# Stars/Sun/Heliocentric astronomy

There are situations of observational astronomy where putting a detector system in orbit around the Sun is a benefit. These efforts are heliocentric astronomy.

## Continua

"It [the Solar Oscillation Imager (SOI) onboard Ulysses] will provide high precision solar images 1024x1024 of line-of-sight velocity, line intensity, continuum intensity, longitudinal magnetic field and limb position." Bold added.

## Emissions

The Coronal Diagnostic Spectrometer (CDS) aboard SOHO "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."

## Acoustics

Global Oscillations at Low Frequencies (GOLF) aboard SOHO "studies the internal structure of the Sun by measuring velocity oscillations over the entire solar disc."
The Michelson Doppler Imager/Solar Oscillations Investigation (MDI/SOI) aboard SOHO 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."

## Particles

The Comprehensive Suprathermal and Energetic Particle Analyzer (COSTEP) aboard SOHO "detects and classifies very energetic particle populations of solar, interplanetary, and galactic origin."

## Cosmic rays

In the graph on the right, the black line is cosmic-ray data and the red line is temperature. Ulysses data is included.
The Charge, Element, and Isotope Analysis System (CELIAS) aboard SOHO "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."

## Neutrals

The Solar Wind Anisotropies (SWAN) aboard SOHO "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’."

## Neutrons

The Neutron Monitor aboard Ulysses was used to measure cosmic rays as well as neutrons.

## Electrons

The Energetic and Relativistic Nuclei and Electron experiment (ERNE) aboard SOHO "measures high-energy particles originating from the Sun and the Milky Way."

## Gamma rays

"Ulysses is launched October 6, 1990, and reached Jupiter for its "gravitational slingshot" in February 1992. It passed the south solar pole in June 1994 and crossed the ecliptic equator in February 1995. The solar X-ray and cosmic gamma-ray burst experiment (GRB) had 3 main objectives: study and monitor solar flares, detect and localize cosmic gamma-ray bursts, and in-situ detection of Jovian aurorae. Ulysses was the first satellite carrying a gamma burst detector which went outside the orbit of Mars. The hard X-ray detectors operated in the range 15–150 keV. The detectors consisted of 23-mm thick × 51-mm diameter CsI(Tl) crystals mounted via plastic light tubes to photomultipliers. The hard detector changed its operating mode depending on (1) measured count rate, (2) ground command, or (3) change in spacecraft telemetry mode. The trigger level was generally set for 8-sigma above background and the sensitivity is 10$^{−6}$ erg/cm$^2$ (1 nJ/m$^2$). When a burst trigger is recorded, the instrument switches to record high resolution data, recording it to a 32-kbit memory for a slow telemetry read out. Burst data consist of either 16 s of 8-ms resolution count rates or 64 s of 32-ms count rates from the sum of the 2 detectors. There were also 16 channel energy spectra from the sum of the 2 detectors (taken either in 1, 2, 4, 16, or 32 second integrations). During 'wait' mode, the data were taken either in 0.25 or 0.5 s integrations and 4 energy channels (with shortest integration time being 8 s). Again, the outputs of the 2 detectors were summed."

## X-rays

"The Ulysses soft X-ray detectors consisted of 2.5-mm thick x 0.5 cm$^2$ area Si surface barrier detectors. A 100 mg/cm$^2$ beryllium foil front window rejected the low energy X-rays and defined a conical FOV of 75° (half-angle). These detectors were passively cooled and operate in the temperature range −35 to −55 °C. This detector had 6 energy channels, covering the range 5–20 keV."

## Opticals

The Variability of Solar Irradiance and Gravity Oscillations (VIRGO) on board SOHO "characterises 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."

## Ultraviolets

The Extreme ultraviolet Imaging Telescope (EIT) aboard SOHO "provides full disc images of the Sun at four selected colours in the extreme ultraviolet, mapping the plasma in the low corona and transition region at temperatures between 80 000 and 2 500 000°C."
The Solar Ultraviolet Measurements of Emitted Radiation (SUMER) instrument on board SOHO "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°C and above."
The UltraViolet Coronograph Spectrometer (UVCS) aboard SOHO "makes measurements in ultraviolet light of the solar corona (between about 1.3 and 12 solar radii from the centre) 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 behaviour of the highly ionised coronal plasma."

## Visuals

Large Angle and Spectrometric Coronograph (LASCO) aboard SOHO "observes the outer solar atmosphere (corona) from near the solar limb to a distance of 21 million kilometres, that is, 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."

## Solar winds

The diagram on the right describes the Solar wind dynamic pressure as detected by Ulysses-SWOOPS.
"The average pressure of the solar wind has dropped more than 20% since the mid-1990s. This is the weakest it's been since we began monitoring solar wind almost 50 years ago."
"Curiously, the speed of the million mph solar wind hasn't decreased much—only 3%. The change in pressure comes mainly from reductions in temperature and density. The solar wind is 13% cooler and 20% less dense."
"Global measurements of solar wind pressure by Ulysses [are shown in the diagram on the right]. Green curves trace the solar wind in 1992-1998, while blue curves denote lower pressure winds in 2004-2008."
"What we're seeing is a long term trend, a steady decrease in pressure that began sometime in the mid-1990s."
"It's hard to say [how unusual this event is]. We've only been monitoring solar wind since the early years of the Space Age—from the early 60s to the present. Over that period of time, it's unique. How the event stands out over centuries or millennia, however, is anybody's guess. We don't have data going back that far."
"Ulysses also finds that the sun's underlying magnetic field has weakened by more than 30% since the mid-1990s."
"Unpublished Ulysses cosmic ray data show that, indeed, high energy (GeV) electrons, a minor but telltale component of cosmic rays around Earth, have jumped in number by about 20%."
"The solar wind streams off of the Sun in all directions at speeds of about 400 km/s (about 1 million miles per hour). The source of the solar wind is the Sun's hot corona. The temperature of the corona is so high that the Sun's gravity cannot hold on to it. Although we understand why this happens we do not understand the details about how and where the coronal gases are accelerated to these high velocities. This question is related to the question of coronal heating."
"The solar wind is not uniform. Although it is always directed away from the Sun, it changes speed and carries with it magnetic clouds, interacting regions where high speed wind catches up with slow speed wind, and composition variations. The solar wind speed is high (800 km/s) over coronal holes and low (300 km/s) over streamers. These high and low speed streams interact with each other and alternately pass by the Earth as the Sun rotates. These wind speed variations buffet the Earth's magnetic field and can produce storms in the Earth's magnetosphere."
"The Ulysses spacecraft completed two orbits through the solar system during which it passed over the Sun's south and north poles. Its measurements of the solar wind speed, magnetic field strength and direction, and composition have provided us with a new view of the solar wind. Ulysses was retired on June 30, 2009."
The second image down on the right shows the results of Ulysses spacecraft measurements of the solar wind speed.

## Interplanetary medium

From model calculations based on data from Ulysses and Skylab, “[i]nside 2 R$_ʘ$ the [interplanetary medium] temperature is a minimum over the poles, with values T$_{eff}$ ~10$^6$ K, while farther from the Sun the temperature is a maximum over the poles with T$_{eff}$ ~3 x 10$^6$ K at its maximum value [at about 5 R$_ʘ$ out to 7 R$_ʘ$].” T$_{eff}$ is estimated to be ~2 x 10$^5$ K at 1 AU over the poles and ~1.5 x 10$^5$ in the equatorial region out at 1 AU, which compare well with spacecraft observations.

## Orbits

Heliocentric orbits can be those approximately independent of the Earth's orbit around the Sun or approximately dependent on the Earth's orbit such as an Earth-trailing, Earth-leading or an Earth-Sun Lagrange point solar orbit.

## Earth-trailing astronomy

"An Earth Trailing Solar Orbit (ETSO)" causes Spitzer "to drift from Earth at a rate of about 0.1 AU per year."
The figure at right shows the Earth-trailing solar orbit (ETSO) for Spitzer with the Earth at the origin and the Sun at left in the rotating coordinate frame "for an 8/25/03 launch projected onto the Ecliptic plane during the 62-month mission lifetime".

## Earth-Sun Lagrange astronomy

The WIND spacecraft spent "several months at the L1 Langrangian point--the point where the gravitational and centrifugal pull of the Sun and Earth cancel each other".
"SOHO [Solar and Heliospheric Observatory] moves around the Sun in step with the Earth, by slowly orbiting around the First Lagrangian Point (L1), where the combined gravity of the Earth and Sun keep SOHO in an orbit locked to the Earth-Sun line. The L1 point is approximately 1.5 million kilometres away from Earth (about four times the distance of the Moon), in the direction of the Sun. There, SOHO enjoys an uninterrupted view of our daylight star."

## WIND spacecraft

"Launched on 1 November 1994 by NASA, the satellite spends most of its time flying into the solar wind (on the sunny side of Earth) in order to decipher its physical and chemical properties."

## Spitzer telescope

"The Spitzer Space Telescope (SST), formerly the Space Infrared Telescope Facility (SIRTF) is an infrared space observatory launched ... from Cape Canaveral Air Force Station, on a Delta II 7920H ELV rocket, Monday, 25 August 2003 at 13:35:39 UTC-5 (EDT)."
"Cryogenic satellites that require liquid helium (LHe, T ≈ 4 K) temperatures in near-Earth orbit are typically exposed to a large heat load from the Earth, and consequently entail large usage of LHe coolant, which then tends to dominate the total payload mass and limits mission life. Placing the satellite in solar orbit far from Earth allowed innovative passive cooling such as the sun shield, against the single remaining major heat source to drastically reduce the total mass of helium needed, resulting in an overall smaller lighter payload, with major cost savings. This orbit also simplifies telescope pointing, but does require the Deep Space Network for communications."

## Helios

"Helios 1 and Helios 2 ... are a pair of probes launched into heliocentric orbit for the purpose of studying solar processes. ... The probes are notable for having set a maximum speed record among spacecraft at 252,792 km/h (157,078 mi/h or 43.63 mi/s or 70.22 km/s or 0.000234c). Helios 2 flew three million kilometers closer to the Sun than Helios 1, achieving perihelion on 17 April 1976 at a record distance of 0.29 AU (or 43.432 million kilometers), slightly inside the orbit of Mercury. Helios 2 was sent into orbit 13 months after the launch of Helios 1. ... The probes are no longer functional but still remain in their elliptical orbit around the Sun."
On board, each probe carried an instrument for cosmic radiation investigation (the CRI) for measuring protons, electrons, and X-rays "to determine the distribution of cosmic rays."

## Ulysses

Ulysses is a "robotic space probe ... designed to study the Sun as a joint venture of NASA and the European Space Agency (ESA)." To obtain an Out-Of-The-Ecliptic (OOE) heliocentric orbit Ulysses swung by Jupiter. Between 1994 and 1995 it explored both the southern (June - October 1994) and northern (June - September 1995) solar polar regions. "Between 2000 and 2001 it explored the southern solar polar regions, which gave many unexpected results. In particular the southern magnetic pole was found to be much more dynamic than the north pole and without any fixed clear location." It operates over the Sun's poles for the third and last time in 2007 and 2008. "After it became clear that the power output from the spacecraft's RTG would be insufficient to operate science instruments and keep the attitude control fuel, hydrazine, from freezing, instrument power sharing was initiated. Up until then, the most important instruments had been kept online constantly, whilst others were deactivated. When the probe neared the Sun, its power-hungry heaters were turned off and all instruments were turned on."
The second down on the right is an artist's impression of Ulysses at the Jupiter H1 point.

## Hypotheses

* Heliocentric astronomy can determine if there is another astronomical object always on the opposite side of the Sun from the Earth.