Image of the Week

Gaia spots a Sunspot

   
 

Figure 1: Full Sun disk images showing sunspot AR12192 as it passed across the face of the Sun in 2014. Credits: NASA/SDO and the AIA, EVE and HMI science teams

 

One of the reasons that the operational orbit at the second Sun-Earth Lagrange point (L2) was selected for Gaia is because it provides the thermal stability that is needed to make the extremely precise measurements of star positions for which the spacecraft was built. Satellites in Earth orbit experience deep eclipses where either the Earth or the Moon passes in front of the Sun and the thermal disturbances caused by these eclipses would affect the Gaia measurements. The Gaia orbit at L2 has been designed to have no Earth transits across the face of the Sun although there are occasional transits of the Moon which cause small thermal changes onboard.

The Gaia spacecraft temperature is passively controlled, this means that there are no switching heaters used to control the temperature, and it is allowed to reach a temperature where the solar heating of the structure is balanced by the blackbody radiation to space. Because of this operational strategy, the temperature of Gaia is sensitive to the received solar power which varies over a year as the distance to the Sun changes slightly due to the ellipticity of the Earth's orbit.

The power of the solar radiation itself is known to be very stable over long periods of time, but it is not constant and has some deviations during an eleven year solar cycle. On a shorter timescale, it has been known for hundreds of years that the visible face of the sun can show sunspots, and it is understood that these sunspots are areas of the photosphere of the Sun which appear to be black when viewed in comparison to the surrounding surface because they are much cooler. Observed from the Earth, these sunspots are seen to move across the face of the Sun in about 13 days due to a combination of the rotation of the Sun and the orbit of the Earth around it.

The Sun has been under continual surveillance from space by a fleet of spacecraft for a few decades. For Gaia, one of the interesting parameters that is collected, because of the potential to effect the thermal balance, is the total solar irradiance (TSI) which is a measure of the solar output power over a wide band of the electromagnetic spectrum. This value is normally quoted at the average distance of the Earth from the Sun and has a value of about 1361 W m-2.

The observed TSI can be influenced by the appearance of a sunspot where the cooler area of the spot emits less energetic photons. The largest sunspot of the current solar cycle, numbered AR12192, was as big as the planet Jupiter and appeared during the early routine operations phase of the Gaia mission in October 2014. Images of the Sun's surface in visible light were acquired at this time by the Solar Dynamics Observatory (SDO) spacecraft (Figure 1). The images of this sunspot show it rotating into view on about 18 October, crossing the centre of the Sun on the 23rd and departing out of view on the 28th of the month. During this time, the sunspot did not appear to evolve, although it did emit a number of X-class flares including a class X3.1 flare on 24 October.

   
 

Figure 2: Total solar irradiance change (top) and sun-shield temperature change measured as sunspot AR12192 passed across the face of the Sun in 2014. Credits TSI data: University of Colorado Laboratory for Space and Atmospheric Physics

 

The TSI data for this period were acquired by the Solar Radiation and Climate Explorer (SORCE) spacecraft. Starting on 18 October, these data show a slow decrease in the TSI over a few days with the lowest TSI value about 3 W m-2 (0.22%) less than the normal reached on 23 October (Figure 2). The reason for this slow decrease in observed output is that as the spot rotates across the spherical surface of the Sun its apparent area increases and therefore so does its influence on the measured solar output.

All temperature changes to the Gaia spacecraft have the potential to influence the scientific products, so engineers at the control centre in Darmstadt were curious to know how much this sunspot had affected the temperatures around the spacecraft. A Gaia temperature monitor (thermistor) located in the spacecraft sunshield was selected for investigation. Although this thermistor is not intended for high precision measurements, because of the stability of the thermal environment, it is possible to calculate long-period running averages that provide high resolution data for slow changes such as the transit of this sunspot. Such computations revealed a cooling of the spacecraft sun-shield by up to 0.15 °C caused by the   sunspot AR12192. This is a small effect but within the resolution of the method.

As expected, similar analyses of the data from the payload, which is strongly thermally isolated from the sun-shield, show that there was no significant effect on the instrument from this sunspot.

Acknowledgement: contribution by Edmund Serpell, a Gaia Operations Engineer working in the Mission Operations Centre at ESA's European Space Operations Centre in Darmstadt, Germany.

[Published: 24/03/2016]

 

Image of the Week Archive

2017
16/08: Preliminary view of the Gaia sky in colour
07/07: Chariklo stellar occultation follow-up
24/04: Gaia reveals the composition of asteroids
20/04: Extra-galactic observations with Gaia
10/04: How faint are the faintest Gaia stars?
24/03: Pulsating stars to study Galactic structures
09/02: Known exoplanetary transits in Gaia data
31/01: Successful second DPAC Consortium Meeting
2016
23/12: Interactive and statistical visualisation of Gaia DR1 with vaex
16/12: Standard uncertainties for the photometric data (in GDR1)
25/11: Signature of the rotation of the galactic bar uncovered
15/11: Successful first DR1 Workshop
27/10: Microlensing Follow-Up
21/10: Asteroid Occultation
16/09: First DR1 results
14/09: Pluto Stellar Occultation
15/06: Happy Birthday, DPAC!
10/06: 1000th run of the Initial Data Treatment system
04/05: Complementing Gaia observations of the densest sky regions
22/04: A window to Gaia - the focal plane
05/04: Hipparcos interactive data access tool
24/03: Gaia spots a sunspot
29/02: Gaia sees exploding stars next door
11/02: A new heart for the Gaia Object Generator
04/02: Searching for solar siblings with Gaia
28/01: Globular cluster colour-magnitude diagrams
21/01: Gaia resolving power estimated with Pluto and Charon
12/01: 100th First-Look Weekly Report
06/01: Gaia intersects a Perseid meteoroid
2015
18/12: Tales of two clusters retold by Gaia
11/11: Lunar transit temperature plots
06/11: Gaia's sensors scan a lunar transit
03/11: Celebrity comet spotted among Gaia's stars
09/10: The SB2 stars as seen by Gaia's RVS
02/10: The colour of Gaia's eyes
24/09: Estimating distances from parallaxes
18/09: Gaia orbit reconstruction
31/07: Asteroids all around
17/07: Gaia satellite and amateur astronomers spot one in a billion star
03/07: Counting stars with Gaia
01/07: Avionics Model test bench arrives at ESOC
28/05: Short period/faint magnitude Cepheids in the Large Magellanic Cloud
19/05: Visualising Gaia Photometric Science Alerts
09/04: Gaia honours Einstein by observing his cross
02/04: 1 April - First Look Scientists play practical joke
05/03: RR Lyrae stars in the Large Magellanic Cloud as seen by Gaia
26/02: First Gaia BP/RP deblended spectra
19/02: 13 months of GBOT Gaia observations
12/02: Added Value Interface Portal for Gaia
04/02: Gaia's potential for the discovery of circumbinary planets
26/01: DIBs in three hot stars as seen by Gaia's RVS
15/01: The Tycho-Gaia Astrometric Solution
06/01: Close encounters of the stellar kind
2014
12/12: Gaia detects microlensing event
05/12: Cat's Eye Nebula as seen by Gaia
01/12: BFOSC observation of Gaia at L2
24/11: Gaia spectra of six stars
13/11: Omega Centauri as seen by Gaia
02/10: RVS Data Processing
12/09: Gaia discovers first supernova
04/08: Gaia flag arrives at ESAC
29/07: Gaia handover
15/07: Eclipsing binaries
03/07: Asteroids at the "photo finish"
19/06: Calibration image III - Messier 51
05/06: First Gaia BP/RP and RVS spectra
02/06: Sky coverage of Gaia during commissioning
03/04: Gaia source detection
21/02: Sky-background false detections in the sky mapper
14/02: Gaia calibration images II
06/02: Gaia calibration image I
28/01: Gaia telescope light path
17/01: First star shines for Gaia
14/01: Radiation Campaign #4
06/01: Asteroid detection by Gaia
2013
17/12: Gaia in the gantry
12/12: The sky in G magnitude
05/12: Pre-launch release of spectrophotometric standard stars
28/11: From one to one billion pixels
21/11: The Hipparcos all-sky map
15/10: Gaia Sunshield Deployment Test
08/10: Initial Gaia Source List
17/09: CU1 Operations Workshop
11/09: Apsis
26/08: Gaia arrival in French Guiana
20/08: Gaia cartoons
11/07: Model Soyuz Fregat video
01/07: Acoustic Testing
21/06: SOVT
03/06: CU4 meeting #15
04/04: DPCC (CNES) 
26/03: Gaia artist impression 
11/02: Gaia payload testing  
04/01: Space flyby with Gaia-like data
2012
10/12: DPAC OR#2. Testing with Planck
05/11: Galaxy detection with Gaia
09/10: Plot of part of the GUMS-10 catalogue
23/07: "Gaia" meets at Gaia
29/06: The Sky as seen by Gaia
31/05: Panorama of BAM clean room
29/03: GREAT school results
12/03: Scanning-law movie
21/02: Astrometric microlensing and Gaia
03/02: BAM with PMTS
12/01: FPA with all the CCDs and WFSs
2011
14/12: Deployable sunshield
10/11: Earth Trojan search
21/10: First Soyuz liftoff from the French Guiana
20/09: Fast 2D image reconstruction algorithm
05/09: RVS OMA
10/08: 3D distribution of the Gaia catalogue
13/07: Dynamical Attitude Model
22/06: Gaia's view of open clusters
27/05: Accuracy of the stellar transverse velocity
13/05: Vibration test of BAM mirrors
18/04: L. Lindegren, Dr. Honoris Causa of the Observatory of Paris
19/01: Detectability of stars close to Jupiter
05/01: Delivery of the WFS flight models
2010
21/12: The 100th member of CU3
17/11: Nano-JASMINE and AGIS
27/10: Eclipsing binary light curves fitted with DPAC code
13/10: Gaia broad band photometry
28/09: Measuring stellar parameters and interstellar extinction
14/09: M1 mirror
 
Please note: Entries from the period 2003-2010 are available in this PDF document.