# Solar Satellites

Post-publication activity

Curator: Brian Dennis

This article briefly covers all past, present, and future solar-dedicated spacecraft plus others with solar instruments that have made or will make significant contributions to solar research. First, we provide a list of all spacecraft with solar physics objectives. Then, for a selection of the major solar missions, we provide the names and capabilities of the instruments and links to papers describing the main scientific results.

## Introduction

As pointed out by Sturrock (1980), it was not surprising that one of the first objects to be studied when space research commenced was the Sun. It is usually the brightest source in the sky at most electromagnetic wavelengths and its proximity to the Earth compared to all other astrophysical objects outside the solar system means that it can be studied in far greater detail. Also, considering the dominance of the Sun and its overwhelming importance for life on Earth, solar space observations have always had high priority. This is clear from the large number of spacecraft that have been dedicated to solar observations or which have carried some solar instrumentation. This great interest is, if anything, increasing as modern civilization relies more and more heavily on space-based technologies and their inherent vulnerabilities to the solar-controlled Space Weather. NASA is investing heavily in its Living With a Star program, and other nations are developing burgeoning space programs with significant solar objectives.

In preparing this article, it quickly became apparent that almost all spacecraft can be considered as solar satellites. All spacecraft must contend with the engineering consequences of the intense solar radiation in space, and even those not in Earth orbit can be considered as satellites in orbit about the Sun. The NSSDC Master Catalog (NMC) lists a total of 6404 spacecraft launched to date, an impossible number to review in this article. Even if we restrict ourselves to spacecraft identified in the NMC with Solar Physics, we still get 188 real spacecraft, starting with Sputnik 2 in 1957 and extending through STEREO A and B launched together in 2006. Space Physics has 632 spacecraft listed and Astronomy has 299. Certainly many of them have instruments making significant solar observations. We provide an edited version of the NMC list in Table 1 of spacecraft that have made or are making significant solar observations useful for research in solar physics.

For the purposes of this article, we have further restricted the number to those spacecraft whose primary objective was, or is, remote sensing of the Sun and those that, in the opinion of the authors, have made, or will make, observations of great significance to the development of solar physics. The list of past, present, and future missions selected is in the table of contents. A brief description is given of each selected mission with links to more detailed information. An attempt has also been made to summarize the major solar physics results from each mission, or at least provide links to published literature. A useful resource for finding publications of results from a given mission is the Aschwanden Solar Literature References Matrix.

After the first flights on captured V-2 rockets had paved the way with early observations in the far ultra-violet (UV) starting in 1946, NASA initiated the Orbiting Solar Observatory (OSO) series of dedicated unmanned solar satellites and undertook the first major manned solar mission from space called Skylab. Together, these early solar programs provided many outstanding observations in the UV, EUV, X-rays, and gamma-rays that provided the foundation of solar space science. Several other nations followed with their own solar missions, either alone as in the case of the Soviet Union or in collaboration with the United States. What follows is a summary of the major solar missions that have made or are expected to make major contributions to our understanding of solar physics though remote sensing.

## Timeline

Figure 1: This plot shows the periods of operation of the major past, present, and future solar space missions. The bars that fade out in 2009 represent missions that are still operational and can be expected to continue as long as the instruments function and funding is available. Future missions are shown starting at their expected launch dates and extending through their planned lifetimes.

## Past Missions

### Orbiting Solar Observatories

Figure 2: OSO 8

Early in the space program, under the direction of John Lindsay, the head of the Solar Physics Branch at Goddard Space Flight Center, NASA initiated the Orbiting Solar Observatory (OSO) series of dedicated unmanned solar satellites. Together with Skylab, the solar instruments carried on these early spacecraft provided outstanding UV and EUV imaging and spectroscopic observations together with X-ray, and gamma-ray spectroscopy. The results of these pioneering observations still, to this day, provide the foundation of much of solar space science.

The OSO series eventually involved a total of nine spacecraft. They were given sequential letter designations (OSO-A through OSO-I) until launch at which point they were renamed with the next number in the sequence. OSO-C was not launched because of a failure during testing on April 14, 1964, that damaged the spacecraft and resulted in the death of 3 engineers. Thus, the eight successful spacecraft were named OSO-1, launched in 1962, through OSO-8, launched in 1975. The first seven OSOs were built by Ball Brothers Research Corporation (later called Ball Aerospace) but Hughes Aircraft won the follow-on contract for OSO-I, J, and K. OSO-I became OSO-8 at launch on 21 June 1975 but OSO-J and -K were combined together for budgetary reasons to become the Solar Maximum Mission (SMM).

As shown in the figure, all the OSO spacecraft incorporated a "wheel" component spinning at up to 15 rpm to provide pointing stability, and a de-spun platform to carry the imaging instruments pointed at the Sun. This arrangement allowed for exquisite sub-arcsecond pointing accuracy and stability before the era of 3-axis stabilization techniques that are used today. Several other instruments that did not need to be constantly and precisely pointed at the Sun were mounted in the spinning "wheel" section. These included X-ray and gamma-ray spectrometers pointed perpendicular to the spin axis so that they scanned across the Sun each rotation, and similar instruments pointed parallel or near -parallel to the spin axis to view non-solar sources of astrophysical interest. Solar panels mounted on the de-spun platform provided the power, and slip-rings between the spinning and de-spun components were used for power and signal connections.

### Skylab

Figure 3: Skylab

The biggest early manned program to study the Sun was Skylab with the Apollo Telescope Mount (ATM) as the main solar observatory. A solar panel and part of its external shielding were lost on launch, and the astronauts had to rig a "golden umbrella" to keep the temperature under control; this is visible in the picture. Skylab operated for six years in orbit from launch on 14 May 1973 through re-entry on 11 July 1979. Skylab carried many astrophysics instruments with eight separate solar instruments on the Apollo Telescope Mount (ATM). These are listed in Table 2. Skylab provided many outstanding observations made primarily during the three manned visits in 1973 when astronauts tended the instruments and returned the film used to record the images. The major results were presented in several reports from a series of workshops held to promote the study of these remarkable observations.

### P78-1

Figure 4: P78-1

This mission was launched on February 24, 1979, as part of the US Department of Defense Space Test Program. The spacecraft was of the Orbiting Solar Observatory (OSO) type, with a solar-oriented sail and a rotating wheel section. In fact, it was built using flight spare components from OSO-7. It continued operating until September 13, 1985, when it was destroyed in orbit during a US Air Force test of an anti-satellite weapon (ASAT) launched from an F-15 fighter plane at 80,000 feet.

The solar instruments included high-resolution Bragg crystal spectrometers covering selected wavelength ranges between 1.82 and 8.53 $$\AA\ ,$$ a spectrometer / spectroheliometer operating between 3 and 25 $$\AA\ ,$$ X-ray proportional-counter spectrometers operating between 1 and 250 keV (12 to 0.05 $$\AA\ ,$$ respectively), and a white light coronagraph.

Scientific returns from this mission include the first observations of a halo coronal mass ejection, and the discovery of the association of mass ejections with interplanetary shocks. High quality and high spectral resolution X-ray spectra of flares and active regions at wavelengths below $$\sim25 \AA$$ were obtained and the signature of chromospheric evaporation on X-ray line profiles was first reported from these observations.

### Solar Maximum Mission (SMM)

Figure 5: SMM

In contrast with all of the OSO spacecraft, the Solar Maximum Mission was 3-axis stabilized so that all of the instruments were pointed at the Sun with arcsecond accuracy and stability. The payload was carefully selected to concentrate on obtaining observations that could further understanding of solar flares. These powerful phenomena had been revealed by earlier observations to be the most energetic explosions in the Solar System but the physical processes involved in the impulsive energy release and subsequent dissipation were largely unknown. To this end, imaging and spectroscopy was carried out at UV, EUV, and X-ray wavelengths to study the emission from plasma heated to temperatures as high as several tens of million kelvin. The electrons accelerated during flares to suprathermal energies were detected through the bremsstrahlung X-rays that they produced. SMM included the first instrument capable of imaging these X-rays up to energies of 30 keV, well above the energies produced by all but the hottest plasma. The protons and heavier ions also accelerated during flares were detected through the nuclear gamma-rays that they generate as they interact in the solar atmosphere. SMM carried a sensitive gamma-ray spectrometer similar to the spectrometer that had made the first pioneering measurements of this high energy emission on OSO-7 back in 1972. Reviews of the scientific results from SMM can be found in the Springer publication, The Many Faces of the Sun : a summary of the results from NASA's Solar Maximum Mission by Strong et al. (1999).

SMM was launched by a Delta rocket on 14 February, 1980, on the rising phase of the 11-year cycle of activity. It operated successfully until November 1980, when the last of several fuses in the aspect control system failed and the spacecraft was no longer able to maintain the orientation towards the Sun with the arcsecond stability that was needed by the imaging instruments. The non-imaging instruments continued to operate as the spacecraft slowly spun about an axis pointed within a few degrees of Sun center. Then in April, 1984, in the first of many dramatic spacecraft rescue missions using the Space Shuttle, the aspect system was fixed by the astronauts, and SMM was able to provide 6 more years of excellent solar observations until it re-entered the Earth's atmosphere in December, 1989.

### Hinotori

Figure 6: Hinotori

The Japanese launched a highly innovative solar dedicated spacecraft called Hinotori (which means "phoenix" in English) on 21 February, 1981 that was to rival SMM, especially in its hard X-ray imaging capability. It was the first mission to use modulation X-ray collimators on a spinning spacecraft to make X-ray images of solar flares using a Fourier-transform technique. This same technique was to be repeated almost 20 years later on NASA's High Energy Solar Spectroscopic Imager (HESSI) later renamed the Reuven Ramaty HESSI (RHESSI). The X-ray images made with Hinotori data at energies above ~25 keV were the first to show clear evidence for footpoint brightening during impulsive flares.

### Ulysses

Figure 7: Ulysses

The joint ESA-NASA Ulysses mission was launched in 1990 into a unique out-of-the-ecliptic orbit that goes over the Sun's poles every 6.2 years.

Figure 8: Ulysses Orbit

It is the only mission capable of overcoming the limitations of all other measurements that are restricted to the vicinity of the ecliptic plane. This unique perspective has provided unprecedented details about the nature of the solar wind. With a nominal mission lifetime of 5 years, Ulysses continued to provide new scientific results until early 2008 when a diminishing power supply lead to the failure of the X-band transmitter and heating systems.

Ulysses carries the following instruments:

• Magnetometer (VHM/FGM)
• Solar Wind Plasma Experiment (SWOOPS)
• Solar Wind Ion Composition Instrument (SWICS)
• Unified Radio and Plasma Wave Instrument (URAP)
• Energetic Particle Instrument (EPAC)
• Interstellar Neutral-Gas Experiment (GAS)
• Low-Energy Ion and Electron Experiment (HISCALE)
• Cosmic Ray and Solar Particle Instrument (COSPIN)
• Solar X-ray and Cosmic Gamma-Ray Burst Instrument (GRB)
• Dust Experiment (DUST)
• Coronal-Sounding Experiment (SCE)
• Gravitational Wave Experiment (GWE)

### Compton Gamma Ray Observatory (CGRO)

Figure 9: CGRO

Although strictly speaking not a solar satellite, CGRO, the second of NASA's Great Observatories, made many important observations of solar flare X-rays and gamma-rays. Launched in April 1991 and re-entering the Earth's atmosphere in June 4 2000, CGRO covered almost a solar cycle with observations spanning an unprecedented six decades of the electromagnetic spectrum, from X-rays as low as 30 keV to gamma rays at 30 GeV. The four instruments are listed in Table 3:

BATSE solar flare data are available at the Solar Data Analysis Center. Over 60 CGRO-related solar publications are listed in the Aschwanden Solar Literature Reference Matrix.

More recent Gamma-ray observations continue to be made with the even more sensitive ESA INTErnational Gamma-Ray Astrophysics Laboratory (INTEGRAL) in cooperation with Russia and the United States. The four instruments on INTEGRAL cover the energy range from 3 keV to 10 MeV. Unfortunately, INTEGRAL cannot be pointed at the Sun, and so flare gamma-rays can only be detected through the thick anticoincidence shield around the high-resolution germanium detectors.

### Yohkoh (Sunbeam in Japanese)

Figure 10: Yohkoh

Yohkoh, the second Japanese solar satellite with US and UK collaboration, was launched on August 30, 1991. All instruments operated successfully for over ten years until December 14, 2001, when the spacecraft failed to recover from a "safehold" resulting from an annular solar eclipse. The four instruments are given in Table 4.

During the over ten years of observations, more that 6 million SXT snapshot images were accumulated, and more than 2,800 flares were detected with HXT and their spectra recorded with BCS and WBS.

Perhaps the most noteworthy scientific result from Yohkoh was the detection of a coronal hard X-ray source in what has become known as the "Masuda Flare." In a rare solar paper published in Nature entitled "A loop-top hard X-ray source in a compact solar flare as evidence for magnetic reconnection," the authors claim to have identified the reconnection region as the site of particle acceleration (Masuda, S., Kosugi, T., Hara, H., Tsuneta, S., and Ogawara, Y., Nature, 371, 495, 1994).

Other major achievements include the establishment of double hard X-ray sources, first observed with the Hard X-ray Imaging Spectrometer on the Solar Maximum Mission, as bremsstrahlung emission from nonthermal electrons accelerated in the corona during a flare and propagating to the footpoints of magnetic loops. Thermal sources at the tops of magnetic loops were also regularly observed in soft X-rays during flares and a super-hot (> 30 MK) thermal component was often seen in hard X-rays during larger flares.

Many scientific results from Yohkoh are summarized in the Nuggets. Reviews of the major results can be found in the proceedings of the Yohkoh 10th Anniversary Meeting. The broad scope and influence of Yohkoh across most of solar physics can be appreciated from the topics included in the different chapters in this proceedings - the quiet Sun, the solar wind, active regions, bright points, flares, CMEs, the solar cycle. Further references can be found in the over 1500 Yohkoh-related publications listed in the Aschwanden Solar Literature Reference Matrix.

References

• Martens & Cauffman, 2002. "Multi-Wavelength Observations of Coronal Structure and Dynamics - Yohkoh 10th Anniversary Meeting. Proceedings of the COSPAR Colloquium held 20-24 January, 2002, held in Kona, Hawaii, USA". Elsevier Science/COSPAR Colloquia Series, v. 13, 2002.

### CORONAS

Figure 11: CORONAS

During this early period of solar space observations, the Russians also launched several spacecraft with instrumentation designed to look primarily at solar flares. Although less technically sophisticated than the NASA instrumentation, they still managed to make useful contributions to solar science.

The Russian CORONAS project (Complex ORbital Observations Near-Earth of Aactivity of the Sun) was envisioned to include three missions to make observations during different phases of the 11-year solar cycle.

CORONAS-I

The first mission in the series, CORONAS-I was launched on 2 March 1994 during solar minimum and re-entered on 4 March 2001. It carried a total of 12 science instruments including the Terek spectroheliometer, the RES-K solar x-ray spectrograph, the Helikon solar gamma-ray detector, the SUVR-SP-C ultraviolet radiometer, the DIFOS optical photometer, and other instruments.

Coronas-F

CORONAS-F was launched on 31 July 2001 near solar maximum and re-entered on 6 December 2005. It carried the following instruments including UV, EUV, X-ray, and gamma-ray spectrometers, radio receivers, and particle counters:

• DIFOS Multichannel Solar Photometer (IZMIRAN, GAO NANU, AIP)
• SPIRIT Full Sun XUV spectroscopy imaging (FIAN, IAS, IAS)
• SRT Solar X-Ray Telescope (FIAN, IAS)
• RES X-Ray Spectroheliograph (FIAN, SAI)
• DIOGENESS X-Ray Spectrometer and Fotometer (CBK PAN, IZMIRAN)
• RESIK X-Ray Spectrometer (CBK PAN, IZMIRAN, MSSL, RAL, NRL)
• SPR Solar Spectropolarimeter (FIAN, NIIYaF)
• IRIS Flare Spectrometer (FTI)
• HELICON Gamma Spectrometer (FTI)
• RPS X-Ray Spectrometer (IKI, MIFI)
• AVS Time-Amplitude Spectrometer (MIFI, NIIYaF)
• SUFR Solar UV Radiometer (IPG)
• VUSS Solar UV Spectrophotometer (IPG)
• SKL Solar Cosmic Rays Complex (NIIYaF, IEP)
• MKL Cosmic Ray Monitor
• SKI Spectrometer of Energy and Ion Chemical Composition
• SONG Solar Neutron and Gamma Ray Spectrometer
• PR-N: X-ray polarimeter
Figure 12: Coronas Photon

CORONAS-PHOTON was the third satellite in the CORONAS series and was launched on 29 January 2009 into a circular, near-polar orbit. All scientific instruments on the satellite were turned off due to power supply problems on 1 December 2009 and the mission was declared to be lost on 18 April 2010.

The main scientific goals of the project were as follows:

• Investigate the energy accumulation and its transformation into energy of accelerated particles during solar flares.
• Study the acceleration mechanisms, propagation and interaction of fast particles in the solar atmosphere.
• Study the solar activity correlation with physical-chemical processes in the Earth's upper atmosphere.

The comprehensive Coronas-Photon payload complex includes the following UV, EUV, X-ray, gamma-ray, and cosmic ray instruments for a total payload weight of 540 kg:

• Natalya-2M spectrometer MIFI, Moscow, Russia
• RT-2 gamma-telescope TIFR/ICSP/VSSC,[14] India.
• Pingvin-M (Penguin) polarimeter MIFI, Moscow, Russia
• Konus-RF x-ray and gamma spectrometer Ioffe Institute, Russia
• BRM x-ray detector MIFI, Russia
• FOKA UV-detector MIFI, Russia
• TESIS telescope/spectrometer FIAN, Russia, with SphinX soft X-ray spectrophotometer, SRC PAS, Poland
• Electron-M-Peska charged particles analyser NIIYaF MGU, Russia
• STEP-F Electron and proton detector Kharkov National University, Ukraine
• SM-8M magnetometer NPP Geologorazvedka/MIFI, Russia

Service systems

• SSRNI science data collection and registration system IKI, Russia
• Radio transmission system and antennas RNII KP, Russia

## Current Missions

### Geosynchronous Operational Environmental Satellite (GOES) Series

Figure 13: GOES 8

The National Oceanic and Atmospheric Administration (NOAA) operates a series of meteorology observing satellites that is a continuation of the Synchronous Meteorological Satellite (SMS) series. This GOES program formally began in 1975 with the launch of the first operational spacecraft, GOES-A, renamed GOES-1 when it reached orbit. The GOES spacecraft circle the Earth in a geosynchronous orbit, which means they orbit close to the equatorial plane of the Earth at a speed matching the Earth's rotation. This allows them to appear to hover continuously over one position on the surface of the Earth at an altitude of 35,800 km (22,300 miles).

In addition to providing systematic, continuous observations of terrestrial weather, GOES also provides observations of space weather through its Space Environment Monitor (SEM) system. The three main components of space weather monitored by GOES are: X-rays, energetic particles, and the magnetic field. The initial series of satellites maintained attitude control by spinning. This is known as "spin-stabilized." With the advent of GOES-8, launched in 1995, the basic platform design became "3-axis stabilized." In 2001 GOES-12 was launched with a new X-ray instrument onboard -- the Solar X-ray Imager (SXI)**. This instrument creates images of the Sun, whereas the original XRS instrument only generated whole disk flux measurements.

• X-ray Sensor (XRS) is an ion chamber that provides whole-Sun X-ray fluxes in the 0.5-to-4 and 1-to-8 Angstrom (Å) wavelength bands. These observations provide a sensitive means of detecting the start and evolution of solar flares. Two bands are measured to allow the hardness of the solar spectrum to be estimated and hence also the temperature and emission measure of the emitting hot flare plasma.
• Energetic Particle Sensor (EPS) is a solid-state detector with pulse-height discrimination to measure proton, alpha-particle, and electron fluxes.
• Magnetometer is a twin-fluxgate spinning sensor that provides measures of three mutually perpendicular components of the Earth's magnetic field.
• Soft X-ray Imager (SXI) (only on GOES-12 and 13) provides a continuous sequence of full-Sun X-ray images at a 1-minute cadence with a 512x512 intensified CCD. Broadband filters are employed to obtain images in several wavelength bands between about 6 and 60 Å.

* GOES-G failed to make it into orbit when its Delta rocket lost control after being struck by lightning shortly after liftoff.

**The SXI instrument is out of commission indefinitely due to a GOES 12 X-ray Sensor (XRS) anomaly on April 12, 2007.

***No GOES 13 SXI images are available after December 2006 due to a detector anomaly that occurred in conjunction with an X-class flare.

References

• Donnelly, R. F. et al. 1977, "Solar X-ray measurements from SMS-1, SMS-2, and GOES-1, information for data users", STIN, 7813992
• Wagner, W. J. et al. 1987, "The Solar X-Ray Imagers (SXI) on NOAA's GOES", BAAS, 19, 923

### Solar and Heliospheric Observatory (SOHO)

Figure 14: The SOHO spacecraft

SOHO is an international collaboration between ESA and NASA to study the Sun from its deep core to the outer corona and the solar wind. It was launched on December 2, 1995, and maneuvered into an orbit about the Earth's First Lagrangian Point (L1). Here, the combined gravity of the Earth and Sun keep SOHO locked near the Earth-Sun line, approximately 1.5 million kilometers from Earth.

Originally designed for a nominal mission lifetime of two years, SOHO has now been in operation for well over a complete 11-year solar cycle. Control of the spacecraft was lost in June, 1998, and only restored three months later. All 12 instruments were still usable, most with no ill effects. However, two of the three on-board gyroscopes failed immediately and a third in December, 1998. This necessitated new on-board software, installed in February 1999, that no longer relies on gyroscopes.

SOHO was designed to study the internal structure of the Sun, its extensive outer atmosphere and the origin of the solar wind, helping us to understand the complex interactions between the Sun and the Earth's environment better than ever before. Among its many accomplishments, SOHO has imaged our star's convective layer and the structure of sunspots beneath the surface; provided profiles of the temperature, rotation, and gas flows in the interior; measured the acceleration of the fast and slow solar wind; discovered new dynamic solar phenomena such as coronal waves and solar tornadoes; revolutionized our ability to forecast space weather; and monitored the total solar irradiance which is important in understanding the impact of solar variability on Earth’s climate. Although not designed for the purpose, SOHO has become the most prolific discoverer of comets in the history of astronomy, identifying over 1500 to date.

• Coronal Diagnostic Spectrometer (CDS) detects emission lines from ions and atoms in the solar corona and transition region, providing diagnostic information on the solar atmosphere, especially of the plasma in the temperature range from 10 000 to more than 1 000 000°C.
• Charge, Element, and Isotope Analysis System (CELIAS) continuously samples the solar wind and energetic ions of solar, interplanetary and interstellar origin, as they sweep past SOHO. It analyzes the density and composition of particles present in this solar wind. It warns of incoming solar storms that could damage satellites in Earth orbit.
• Comprehensive Suprathermal and Energetic Particle Analyzer (COSTEP) detects and classifies very energetic particle populations of solar, interplanetary, and galactic origin. It is a complementary instrument to ERNE (see below).
• Extreme ultraviolet Imaging Telescope (EIT) provides full disc images of the Sun at four selected wavelengths in the extreme ultraviolet, mapping the plasma in the low corona and transition region at temperatures between 80 000 and 2 500 000 Kelvin.
• Energetic and Relativistic Nuclei and Electron experiment (ERNE) measures high-energy particles originating from the Sun and the Milky Way. It is a complementary instrument to COSTEP.
• Global Oscillations at Low Frequencies (GOLF) studies the internal structure of the Sun by measuring velocity oscillations over the entire solar disc.
• Large Angle and Spectrometric Coronograph (LASCO) observes the outer solar atmosphere (corona) from near the solar limb to a distance of 21 million kilometers (about one seventh of the distance between the Sun and the Earth). LASCO blocks direct light from the surface of the Sun with an occulter, creating an artificial eclipse, 24 hours a day, 7 days a week. LASCO has also become SOHO’s principal comet finder.
• Michelson Doppler Imager/Solar Oscillations Investigation (MDI/SOI) records the vertical motion (“tides”) of the Sun's surface at a million different points for each minute. By measuring the acoustic waves inside the Sun as they perturb the photosphere, scientists can study the structure and dynamics of the Sun’s interior. MDI also measures the longitudinal component of the Sun’s magnetic field.
• Solar Ultraviolet Measurements of Emitted Radiation (SUMER) instrument is used to perform detailed spectroscopic plasma diagnostics (flows, temperature, density, and dynamics) of the solar atmosphere, from the chromosphere through the transition region to the inner corona, over a temperature range from 10 000 to 2 000 000 Kelvin and above.
• Solar Wind Anisotropies (SWAN) instrument is the only remote sensing instrument on SOHO that does not look at the Sun. It watches the rest of the sky, measuring hydrogen that is ‘blowing’ into the Solar System from interstellar space. By studying the interaction between the solar wind and this hydrogen gas, SWAN determines how the solar wind is distributed. As such, it can be qualified as SOHO’s solar wind ’mapper’.
• UltraViolet Coronograph Spectrometer (UVCS) makes measurements in ultraviolet light of the solar corona (between about 1.3 and 12 solar radii from the center) by creating an artificial solar eclipse. It blocks the bright light from the solar disc and allows observation of the less intense emission from the extended corona. UVCS provides valuable information about the microscopic and macroscopic behavior of the highly ionised coronal plasma.
• Variability of Solar Irradiance and Gravity Oscillations (VIRGO) instrument characterizes solar intensity oscillations and measures the total solar irradiance (known as the ‘solar constant’) to quantify its variability over periods of days to the duration of the mission.

References

• Domingo, V.et al. 1995, "The SOHO Mission: an Overview", Sol. Phys, 162, 1-37
• Gabriel, A. H. et al. 1995, "Global Oscillations at Low Frequency from the SOHO Mission (GOLF)", Sol. Phys, 162, 61-99
• Fröhlich, C. et al. 1995, "VIRGO: Experiment for Helioseismology and Solar Irradiance Monitoring", Sol. Phys, 162, 101-128
• Scherrer, P. H. et al. 1995, "The Solar Oscillations Investigation - Michelson Doppler Imager", Sol. Phys, 162, 129-188
• Wilhelm, K. et al. 1995, "SUMER - Solar Ultraviolet Measurements of Emitted Radiation", Sol. Phys, 162, 189-231
• Harrison, R. A. et al. 1995, "The Coronal Diagnostic Spectrometer for the Solar and Heliospheric Observatory", Sol. Phys, 162, 230-299
• Delaboudinière, J.-P. et al. 1995, "EIT: Extreme-Ultraviolet Imaging Telescope for the SOHO Mission", Sol. Phys, 162, 291-312
• Kohl, J. L. et al. 1995, "The Ultraviolet Coronagraph Spectrometer for the Solar and Heliospheric Observatory", Sol. Phys, 162, 313-356
• Brueckner, G. E. et al. 1995, "The Large Angle Spectroscopic Coronagraph (LASCO)", Sol. Phys, 162, 357-402
• Bertaux, J. L. et al. 1995, "SWAN: A Study of Solar Wind Anisotropies on SOHO with Lyman Alpha Sky Mapping", Sol. Phys, 162, 403-439
• Hovestadt, D. et al. 1995, "CELIAS - Charge, Element and Isotope Analysis System for SOHO", Sol. Phys, 162, 441-481
• Müller-Mellin, R. et al. 1995, "COSTEP - Comprehensive Suprathermal and Energetic Particle Analyser", Sol. Phys, 162, 483-504
• Torsti, J. et al. 1995, "Energetic Particle Experiment ERNE", Sol. Phys, 162, 505-531

### Transition Region And Coronal Explorer (TRACE)

Figure 15: The TRACE spacecraft

TRACE is designed to make quantitative observations showing the connections between fine-scale magnetic fields and the associated plasma structures in the solar atmosphere. This is achieved by making precisely co-aligned and near simultaneous (within a few seconds) images at the UV and extreme UV wavelengths listed in Table 6. The emission in each wavelength band arises from plasma at different temperatures ranging from 4,000 Kelvin to 4 million Kelvin that correspond to different layers of the solar atmosphere from the photosphere, up through the chromosphere and transition region, and into the corona. The images are recorded on a 1024 x 1024 CCD with a field of view of 8.5 x 8.5 arcminutes and an angular resolution is one arcsecond, corresponding to about 725 kilometers spatial resolution on the Sun.

TRACE was launched into a Sun-synchronous orbit on April 2, 1998, giving it uninterrupted viewing of the Sun for up to eight months at a time.

References

• Handy, B. N. et al. 1999, "The Transition Region And Coronal Explorer", 187, 229-260

### Reuven Ramaty High-Energy Solar Spectroscopic Imager (RHESSI)

Figure 16: RHESSI

RHESSI is a NASA Small Explorer (SMEX) launched on 5 February 2002 to study solar flares through X-ray and gamma-ray imaging spectroscopy observations (Lin et al. 2002). RHESSI's primary mission is to explore the basic physics of particle acceleration and explosive energy release in solar flares. Based on previous observations, scientists believe that much of the energy released during a flare is used to accelerate, to very high energies, electrons (emitting primarily X-rays) and protons and other ions (emitting primarily gamma rays). The new approach of the RHESSI mission is to combine, for the first time, high-resolution imaging in hard X-rays and gamma rays with high-resolution spectroscopy, so that a detailed energy spectrum of the radiation can be obtained at each point of the image.

The RHESSI mission consists of a spin-stabilized spacecraft in a low-altitude orbit inclined at 38 degrees to the Earth's equator. The only instrument on board is an imaging spectrometer that allows high fidelity color movies to be made of solar flares in X-rays and gamma-rays between 3 keV and 17 MeV. This advanced capability is achieved by combining two complementary technologies -

• Fourier-transform imaging (Hurford et al. 2002) using the spacecraft rotation at ~15 rpm and pairs of fine tungsten grids to modulate the solar X-ray and gamma-ray flux, and
• High-resolution spectroscopy using cooled germanium detectors (Smith et al. 2002) to very precisely measure the energy of each photon that passes through the grids.

RHESSI has been operating successfully since launch and now has over 40,000 events listed in its flare catalog. Over 11,000 flares have been identified with detectable emission above 12 keV, ~950 above 25 keV, and 30 above 300 keV, with 18 showing gamma-ray line emission. A summary of the recent scientific results is given in the 2008 NASA Senior Review proposal. Further details can be found in the over 700 RHESSI-related publications listed in the Aschwanden Solar Literature Reference Matrix.

Following the NASA Senior Review recommendations, RHESSI has been approved for continuing operations for another two years. The germanium detectors were successfully annealed in November 2007 to remove the effects of radiation damage, thus restoring the detector energy resolutions and sensitive volumes to usable levels. Further anneals are planned as needed to maintain acceptable detector performance. Since the mission was designed with no expendables and the orbit decay has been minimal, RHESSI should be able to operate for years to come. Thus, RHESSI is ready for the rise from solar minimum to the expected maximum of the next activity cycle in 2009-2012.

References

• Lin, R. P., et al., 2002, "The Reuven Ramaty High-Energy Solar Spectroscopic Imager (RHESSI)", Sol. Phys., 210, 3-32.
• Smith, D. M., et al., 2002, "The RHESSI Spectrometer", Sol. Phys., 210, 33-60.
• Hurford, G. J., et al., 2002, "The RHESSI Imaging Concept", Sol. Phys., 210, 61-86.

### Solar Radiation and Climate Experiment (SORCE)

Figure 17: SORCE

The Solar Radiation and Climate Experiment (SORCE) is a NASA-sponsored satellite mission that provides measurements of incoming X-ray, ultraviolet, visible, near-infrared, and total solar radiation. The measurements provided by SORCE specifically address long-term climate change, natural variability and enhanced climate prediction, and atmospheric ozone and UV-B radiation. These measurements are critical to studies of the Sun, its effect on our Earth system and its influence on humankind. SORCE was launched on January 25, 2003, by a Pegasus XL launch vehicle into a near-circular orbit with an altitude of 645 km and an inclination of 40 degrees. It is operated by the Laboratory for Atmospheric and Space Physics (LASP) at the University of Colorado (CU) in Boulder, Colorado, USA.

SORCE carries the following four instruments:

• Spectral Irradiance Monitor (SIM) is a newly designed spectrometer that provides the first long-duration solar spectral irradiance measurements in the visible and near infrared (Vis/NIR). The wavelength coverage is primarily from 300 to 2400 nm, with an additional channel to cover the 200-300 nm ultraviolet spectral region to overlap with the SOLSTICE instrument.
• SOLar STellar Irradiance Comparison Experiment (SOLSTICE) makes daily solar ultraviolet (115-320 nm) irradiance measurements and compares them to the irradiance from an ensemble of 18 stable early-type stars. This approach provides an accurate monitor of instrument in-flight performance and provides a basis for solar-stellar irradiance comparison for future generations.
• Total Irradiance Monitor (TIM) measures the total solar irradiance (TSI), the integrated solar radiation incident at the top of the Earth's atmosphere. The TIM continues this climate record, which began in 1978 and is used to determine the sensitivity of the Earth's climate to the natural effects of solar forcing.
• XUV Photometer System (XPS), which evolved from earlier versions flown on SNOE and TIMED, continue on these solar XUV irradiance measurements with improvements to accuracy, spectral image, and temporal change.

References

• Rottman, G. 2005, "The SORCE Mission", Sol. Phys, 230, 7-25
• Woods, T N., & Rottman, G. 2005, "XUV Photometer System (XPS): Solar Variations during the SORCE Mission", Sol. Phys, 230, 375-387

### Hinode

Figure 18: Hinode

Hinode is a joint Japan/US/UK mission (formerly known as Solar-B) led by the Japanese Aerospace Exploration Agency's (JAXA) Space Science Research Division (formerly the Institute of Space and Astronautical Science (ISAS)). It is designed to explore the magnetic fields in the solar corona and to improve our understanding of the mechanisms that power the solar atmosphere and drive solar eruptions. Following the JAXA tradition, the mission was renamed after the spacecraft’s first successful orbit from Solar-B to Hinode, a Japanese word that means sunrise. Hinode was launched from Japan's Uchinoura Space Center on September 22, 2006, into a Sun‑synchronous orbit at an altitude of about 600 km, which means that it observes the Sun continuously for nine months at a time. Around the summer (northern hemisphere) solstice each year, Hinode experiences an "eclipse season" during which the Sun is eclipsed by Earth for a maximum of ten minutes in each 98 minute orbit.

Hinode consists of a coordinated set of optical, EUV, and X-ray instruments to investigate the interaction between the Sun's magnetic field and its corona. The three complementary instruments work together as a highly innovative solar observatory.

• The Solar Optical Telescope/Focal Plane Package (SOT/FPP) is the first large optical telescope flown in space. Its aperture is 50 cm in diameter, the angular resolution is 0.25" (corresponding to 175 km on the Sun), and the wavelengths covered extend from 480 to 650 nm. SOT also includes the Focal Plane Package (FPP) that consists of a vector magnetograph and a spectrograph. The vector magnetograph provides time series of photospheric vector magnetograms, Doppler velocity and photospheric intensity.
• The Extreme-ultraviolet Imaging Spectrometer (EIS) is a two-channel, normal-incidence EUV spectrometer. Its two channels cover the wavelength ranges 170-210 Å and 250-290 Å, selected to cover solar coronal emission lines. It has a mirror that is tiltable in the Solar X direction, and is used to build up rastered spectral images of the Sun in up to 25 spectral ranges. Additionally, EIS has both narrow (one- and two-arcsecond wide) slits, and wider (40- and 266-arcsecond) imaging slots, all with 512 arcseconds in the Solar Y direction. Under nominal conditions, the 40-arcsecond slot can be used to make simultaneous, separated, quasi-monochromatic images in up to twelve strong emission lines covering a temperature range from He II (8,000 Kelvin) to Fe XXIV (16 000 000 Kelvin). EIS can make slit images of active regions in 10 s, of the quiet Sun in 30 to 60 s, and of flares in 1 s.
• The X-Ray Telescope (XRT) is a high-resolution (1 arcsecond) grazing-incidence Wolter telescope that is the successor to the highly successful Soft X-Ray Telescope (SXT) on Yohkoh. High-resolution soft X-ray images covering the energy range from 0.2 to 2 keV reveal magnetic field configurations and their evolution, allowing the observation of energy buildup, storage and release process in the corona for any transient event. XRT covers a wide temperature range from 0.5 to 10 million Kelvin allowing it to see all the coronal features that are not all visible with any normal incidence telescope.

References

• Kosugi, T. et al. 2007, "The Hinode (Solar-B) Mission: An Overview", Sol. Phys, 243, 3-17
• Culhane, J. L. et al. 2007, "The EUV Imaging Spectrometer for Hinode", Sol. Phys, 243, 19-61
• Golub, L. et al. 2007, "The X-Ray Telescope (XRT) for the Hinode Mission", Sol. Phys, 243, 63-86
• Tsuneta, S. et al. 2008, "The Solar Optical Telescope for the Hinode Mission: An Overview", Sol. Phys, 249, 167-196

### Solar Terrestrial Relations Observatory (STEREO)

Figure 19: STEREO A and B

STEREO (Solar TErrestrial RElations Observatory) is the third mission in NASA's Solar Terrestrial Probes program (STP). It is designed to view the three-dimensional (3D) and temporally varying heliosphere by means of an unprecedented combination of imaging and in situ experiments mounted on virtually identical spacecraft flanking the Earth in its orbit about the Sun. This two-year mission employs two nearly identical observatories - one ahead of Earth aptly called STEREO-A, the other trailing behind called STEREO-B. Together, they provide the first-ever stereoscopic measurements to study the Sun and the nature of its coronal mass ejections (CMEs). The primary goal of the STEREO mission is to advance our understanding of the three-dimensional structure of the Sun's corona, especially regarding the origin of CMEs, their evolution in the interplanetary medium, and the dynamic coupling between CMEs and the Earth environment. The two STEREO spacecraft were launched together on a Delta II rocket from Cape Canaveral on October 25, 2006, and used a gravity assist from the Moon to slingshot the spacecraft into a heliocentric orbit. STEREO-A went into a heliocentric orbit first followed two weeks later by STEREO-B. The two spacecraft each drift away from Earth at an average rate of about 22.5 degrees per year. Thus, after the two year nominal operations phase, the spacecraft will be about 90 degrees apart, each about 45 degrees from Earth. Since STEREO-A is traveling faster than Earth around the Sun, it has an orbit slightly closer to the Sun than Earth's. Similarly, the STEREO-B is traveling more slowly than Earth and must have an orbit slightly further than Earth. This difference in distance from the Sun must be carefully taken into account in any reconstruction of the 3D features of the solar corona and CME structures.

The spacecraft bus consists of six operational subsystems supporting two instruments and two instrument suites for a total of 16 instruments per observatory. These are briefly described below.

The Sun-Earth Connection and Coronal and Heliospheric Investigation (SECCHI) is a suite of 5 scientific telescopes (EUVI, COR1, COR2, HI1 and HI2) that observes the solar corona and inner heliosphere from the surface of the Sun to the orbit of Earth. SECCHI is named after one of the first astrophysicists, Angelo Pietro Secchi (1818-1878). Angelo Secchi was a Jesuit priest and was one of the first astrophysicists to use the new medium of photography to record solar eclipses. He photographed the 1860 eclipse, during which a CME is now thought to have occurred.

• Coronagraphs COR1 and COR2 observe the inner (1.4 - 4 $$R_{Sun}$$) and outer (2 - 15 $$R_{Sun}$$) corona with greater frequency and polarization precision than ever before. COR1 is the first space borne instrument to explore the inner corona in white light and polarized brightness (pB) down to 1.4 $$R_{Sun}\ .$$ COR2 images the corona with five times the spatial resolution and three times the temporal resolution of LASCO/C3.
• The Extreme Ultraviolet Imager (EUVI) provides full-Sun coverage out to 1.7 $$R_{Sun}$$ with twice the spatial resolution and dramatically improved cadence over SOHO/EIT. EUVI observes the photospheric magnetic field, chromosphere, and innermost corona underlying the same portions of the corona and the heliosphere observed by COR1, COR2, and HI.
• The Heliospheric Imager (HI) is the most novel instrument. It extends the concept of traditional externally occulted coronagraphs to a new regime, the heliosphere from the Sun to the Earth (12-318 $$R_{Sun}$$). HI has obtained the first direct imaging observations of coronal mass ejections in interplanetary space.
• The Guide Telescope acts as a fine Sun sensor for EUVI and provides the error signal for the EUVI fine pointing system.

STEREO WAVE (SWAVES) is an interplanetary radio burst tracker that traces the generation and evolution of traveling radio disturbances from the Sun to the orbit of Earth.

In-situ Measurements of Particles and CME Transients (IMPACT) samples the 3-D distribution and provides plasma characteristics of solar energetic particles and the local vector magnetic field. IMPACT is a suite of seven instruments, three of which are located on a 6-m deployable boom, with the others located on the main body of the spacecraft. The boom suite includes the following instruments:

• The Solar Wind Experiment (SWEA) measures ~0.2 to 1 keV electrons with wide angle coverage,
• The Suprathermal Electron Telescope (STE) measures electrons from 5 to 100 keV with wide-angle coverage.
• The Magnetometer Experiment (MAG) measures the vector magnetic fields in the range of ± 512 nT with 0.1 nT accuracy.
• The Solar Energetic Particle (SEP) suite in the main body includes:
• The Suprathermal Ion Telescope (SIT)
• The Solar Electron and Proton Telescope (SEPT)
• The Low Energy Telescope (LET)
• The High Energy Telescope (HET)

The Plasma and SupraThermal Ion and Composition (PLASTIC) provides plasma characteristics of protons, alpha particles and heavy ions. This instrument provides key diagnostic measurements of the form of mass and charge state, and composition of heavy ions, and characterize the CME plasma from ambient coronal plasma. PLASTIC incorporates three science sensor functions into one package:

• Solar Wind Sector (SWS) Proton Channel measures the distribution functions of solar wind protons and alphas, providing proton density, velocity, kinetic temperature and its anisotropy, and alpha to proton ratios with a time resolution up to about 1 minute.
• Solar Wind Sector (SWS) Main (Composition) Channel measures the elemental composition, charge state distribution, kinetic temperature, and speed of the more abundant solar wind heavy ions (e.g., C, O, Mg, Si, and Fe) by using Electrostatic Analyzer (E/Q), Time-of-Flight (TOF), and Energy (E) measurement to determine Mass and M/Q. Typical time resolution for selected ions will be ~ 5 minutes.
• Wide-Angle Partition (WAP) measures distribution functions of suprathermal ions, including Inter-Planetary Shock-accelerated (IPS) particles associated with CME-related SEP events, recurrent particle events associated with Co-rotating Interaction Regions (CIRs), and heliospheric pickup ions. Typical time resolution for selected ions will be several minutes to hours.

References

• Kaiser, M. L. et al. 2008, "The STEREO Mission: An Introduction", SSRv, 136, 5-16
• Howard, R. A. et al. 2008, "Sun Earth Connection Coronal and Heliospheric Investigation (SECCHI)", SSRv, 136, 67-115
• Luhmann, J. G. et al. 2008, "STEREO IMPACT Investigation Goals, Measurements, and Data Products Overview", SSRv, 136, 117-184
• Bougeret, J. L. et al. 2008, "S/WAVES: The Radio and Plasma Wave Investigation on the STEREO Mission", SSRv, 136, 487-528

### Solar Dynamics Observatory (SDO)

Figure 20: The Solar Dynamics Observatory (SDO)

Launched on February 11, 2010, the Solar Dynamics Observatory is the first mission in NASA's Living With a Star (LWS) Program, a program designed to understand the causes of solar variability and its impacts on Earth. SDO is designed to help understand the Sun's influence on Earth and Near-Earth space by studying the solar atmosphere on small scales of space and time and in many wavelengths simultaneously.

SDO is a sun-pointing semi-autonomous spacecraft, 4.5 meters high and over 2 meters on each side, weighing a total of 3100 kg (fuel included). The inclined geosynchronous orbit was chosen to allow both continuous observations of the Sun and a continuous and exceptionally high data downlink rate of 130 Megabits per second (Mbps) through the use of a single dedicated ground station.

SDO has the following three scientific instruments:

• Atmospheric Imaging Assembly (AIA) images the solar atmosphere in multiple wavelengths to link changes in the surface to interior changes. Data include images of the Sun in 10 wavelengths every 12 seconds.
• EUV Variability Experiment (EVE) measures the solar EUV irradiance with unprecedented spectral resolution, temporal cadence, and precision. Measures the solar extreme ultraviolet spectral irradiance to understand variations on the timescales which influence Earth's climate and near-Earth space.
• Helioseismic and Magnetic Imager (HMI) extends the capabilities of the SOHO/MDI instrument with continuous full-disk coverage at higher spatial resolution.

## Future Missions

### Solar Probe+

Figure 21: Solar Probe+

Solar Probe+ (pronounced Solar Probe Plus) will be a unique mission, exploring what is arguably the last region of the solar system to be visited by a spacecraft, the Sun’s outer atmosphere or corona. Solar Probe+ will sample the near-Sun environment, substantially increasing our knowledge and understanding of coronal heating and of the origin and evolution of the solar wind, and answering some of the most fundamental questions remaining in heliophysics -

• Why is the solar corona so much hotter than the photosphere?
• And how is the solar wind accelerated?

A mission to provide answers to these questions by probing the near-Sun particles and fields environment was first recommended in 1958, at the dawn of the space age, by the National Academy of Science’s “Simpson Committee.” Since then, NASA has conducted several studies of possible implementations of a Solar Probe mission, and a Solar Probe has remained at the top of various National Academy and NASA science priority lists ever since.

The Solar Probe+ major science objectives remain the same as those established for Solar Probe 2005. The four major science objectives are as follows:

1. Determine the structure and dynamics of the magnetic fields at the sources of the fast and slow solar wind.
2. Trace the flow of energy that heats the corona and accelerates the solar wind.
3. Determine what mechanisms accelerate and transport energetic particles at the Sun and in the inner heliosphere.
4. Explore dusty plasma phenomena in the near-Sun environment and their influence on the solar wind and energetic particle formation.

Solar Probe+ differs dramatically from the 2005 design and from all previous Solar Probe mission designs since the 1970s. The earlier designs involved one or two flybys of the Sun at a perihelion distance of 4 solar radii ($$R_{Sun}$$) by a spacecraft placed into a solar polar orbit by means of a Jupiter gravity assist. In contrast, Solar Probe+ remains nearly in the ecliptic plane and makes many near-Sun passes at increasingly lower perihelia. The baseline mission provides for 24 perihelion passes inside 0.16 Astronomical Units (AU) or 35 $$R_{Sun}\ ,$$ with 19 passes occurring within 20 $$R_{Sun}\ .$$ The first near-Sun pass occurs 3 months after launch at a heliocentric distance of 35 $$R_{Sun}\ .$$ Over the next several years successive Venus gravity assist (VGA) maneuvers gradually lower the perihelion to ~9.5 $$R_{Sun}\ ,$$ by far the closest any spacecraft has ever come to the Sun. The spacecraft completes its nominal mission with three passes, separated by 88 days, at this distance.

Solar Probe+ is currently scheduled for launch in 2015.

Reference: [http://solarprobe.gsfc.nasa.gov/SolarProbe+Web.pdf Solar Probe Plus: Report of the Science and Technology Definition Team]

### Solar Orbiter

Figure 22: Solar Orbiter

By approaching as close as 48 solar radii ($$R_{Sun}$$), the Solar Orbiter will view the solar atmosphere with unprecedented spatial resolution. Over extended periods the Solar Orbiter will deliver images and data of the polar regions and the side of the Sun not visible from Earth. Following launch in 2017, Solar Orbiter will begin its journey to the Sun which will require a cruise phase lasting approximately 3.4 years. During this time, the spacecraft will use gravity assists from Venus and the Earth to achieve an orbit with a period of 150 days from which it can begin its scientific mission.

Solar Orbiter is a specially designed three-axis stabilised spacecraft. The smallest face always points to the Sun so the instruments are protected by a sunshield. At closest approach to the Sun, Solar Orbiter will receive 25 times the solar radiation per square meter that the Earth does. The spacecraft will be kept cool by the positioning special 'radiators' to dissipate excess heat into space.

All Solar Orbiter's science instruments are currently in the concept phase. They will however be divided into three packages:

• Field Package:  Radio and Plasma Wave Analyser and Magnetometer.
• Particle Package:  Energetic Particle Detector, Dust Detector and Solar Wind Plasma Analyser.
• Solar Remote Sensing Instrumentation:  Visible-light Imager and Magnetograph, EUV Imager and Spectrometer, EUV Imager, Coronagraph, and Spectrometer/Telescope for Imaging X-rays, as well as either a Generic Heliospheric Imager or a Wide Angle Coronagraph.

The payload will be commissioned during the cruise phase and will already be able to provide good science data. Upon entering the science orbit, closer encounters to the Sun will be achieved. Subsequent Venus gravity assist maneuvers will increase the inclination to the ecliptic plane to provide a better view of the Sun's polar regions.

### Solar Sentinels

Figure 23: Solar Sentinels

Sentinels is the NASA component of the joint NASA–ESA Heliophysical Explorers (HELEX) mission that also includes ESA’s Solar Orbiter.

The Sentinels mission will consist of four identically instrumented, spinning spacecraft that observe the energetic particle environment and solar wind structures in the inner heliosphere, near the ecliptic plane. The spacecraft operate at close distances to the Sun, inside the orbit of Mercury. Sentinels will investigate and characterize how the Sun determines the environment of the inner solar system, and, more broadly, reveal how the heliosphere is formed.

To answer some of the outstanding questions about the solar origins of the heliospheric plasma and energetic particle environment, in situ measurements of the solar wind plasma, fields, waves, and SEPs close enough to the Sun are needed so that they are still relatively unprocessed. The Sentinels are designed with the following major science objectives:

• Make simultaneous in situ measurements at multiple locations to capture the spatial structure and temporal evolution of the phenomena being observed.
• Trace the magnetic field from the locations where the in situ measurements are made back to the source regions on the Sun.
• Perform imaging and spectroscopic observations of the source regions on the Sun simultaneously with the in situ measurements.

Originally intended to be launched in 2016 to complement Solar Orbiter, Sentinels are currently on hold pending the completion of Solar Probe+.

### Solar-C

Solar-C is the name of the next Japanese solar mission following Yohkoh (Solar-A) and Hinode (Solar-B).

There are currently two possible scientific objectives for Solar-C, now scheduled for launch in 2016.

• Plan A: Out-of-ecliptic magnetic, X-ray, and helioseismic observations of the polar and equatorial regions of the Sun. A major objective is to determine the meridional flow and magnetic structure inside the Sun to the base of the convection zone.
• Plan B：High spatial resolution, high throughput, high cadence spectroscopic, polarimetric, and X-ray observations to seamlessly cover the solar atmosphere from the photosphere to the corona. A major objective is to investigate the magnetism of the Sun and to determine its role in the heating and dynamics of the solar atmosphere.

## Bibliography

Internal references

• Sturrock, P. (1980) Solar flares: A monograph from SKYLAB Solar Workshop II. Colorado Associated University Press
• Strong, Keith T.; Saba, Julia L. R.; Haisch, Bernhard M.; Schmelz, Joan T. (1999) The many faces of the sun : a summary of the results from NASA's Solar Maximum Mission. Springer
• Aschwanden, M. (2005) Physics of the Solar Corona. Springer
• Poppe, Barbara B.; Jorden, Kristen P. (2006) Sentinels of the Sun. Johnson