The L2 environment (and orbits around it) is relatively benign compared to those in geostationary (GO), or low Earth (LEO) orbits. In particular, a series of common threats for satellites in GO or LEO, including the neutral thermosphere, space debris, geomagnetically trapped particles and large temperature gradients, are not a concern for L2 orbits. Environmental aspects to be considered at L2 include:
Solar wind plasma. Essentially a neutral or cold plasma: 95% protons, 5% He++ and equivalent electrons; 1-10 particles/cm3. The main risk associated is a low surface charging potential. This plasma may be relatively benign at L2 compared to that found at GO and LEO.
Ionising radiation: solar flares (energetic electrons, protons and alpha particles), Galactic cosmic rays and Jovian electrons.
Magnetic fields: Earth's magnetotail extends up to 1000 Earth's radii, so it must be considered (2-10 nT) along with interplanetary magnetic field (∼ 5 nT). The effects on the spacecraft and PLM include possible orbit disturbance and electrostatic discharge (ESD).
Therefore, the main radiation components at L2 consist of: Galactic cosmic rays, solar particle events and solar and Jovian electrons.
In the early stages of the mission, the dominant radiation source was Jovian electrons, characterised by a energetic population and a 13-month synodic year modulation.
The Herschel spacecraft is equipped with a standard radiation environment monitor (SREM) placed in the -Z SVM panel; the SREM is a particle detector developed for satellite applications that has been added to Herschel and Planck as a passenger. It measures high-energy electrons (from 0.5 MeV to infinity) and protons (from 20 MeV to infinity) of the space environment with an angular resolution of some 20 degrees, providing particle species and spectral information. The SREM data are received on-ground and processed by the Space Weather Group at ESTEC, providing valuable information on the radiation environment at L2. A sample plot showing the calibrated count rates in three counters (TC1 - protons with E > 20 MeV; TC2 - protons with E > 39 MeV; TC3 - electrons with E > 0.5 MeV) is displayed in Figure 4.3 .
Figure 4.3. SREM calibrated count rates in three counters (TC1, TC2 and TC3), rebinned in intervals of five minutes. from the 30th of October 2009 (OD 170) to 27th March of 2011 (OD 683). The slight decline of the count rates can be explained by an increased solar activity and the subsequent increase of shielding to Galactic cosmic rays. Several events are visble, the most conspicuous a small proton flare detected in OD 663 (7-8 March 2011).
Weekly, calibrated plots of the Herschel SREM data and special plots of any observed proton events are available to users, provided by the SREM PI, Petteri Nieminen. These are available at http://proteus.space.noa.gr/~srem/herschel/.
Solar activity follows an approximately 11-year cycle. The last minimum occurred in December 2008 and therefore the Herschel launch in 2009 happened during a low activity state. Contrary to initial pre-launch predictions, the current solar cycle will be below average in intensity, with a predicted maximum sunspot number of 90. Given the predicted date of solar minimum and the predicted maximum intensity, solar maximum is now expected to occur in May, 2013 (Solar Cycle 24 Prediction Panel agreement on May 8, 2009). Therefore Solar particle events are expected to be problematic only towards the end of the mission. As of July 2012, the solar activity is increasing, with proton events of around 6000 proton flux units observed in January and March 2012 being the strongest ones observed to date, while the previous solar maximum gave two events around 30000 proton flux units and events over 40000 proton flux units have been observed since 1976 when solar proton activity records began.
|The amplitude of solar proton storms is measured in "proton flux units" (pfu) at >10 MeV, where 1 pfu is 1 proton cm-2 s-1 sr-1. The threshold for a storm is 10 pfu. In the literature the storm intensity is often given as the integer part of the logarithm of the peak flux in pfu (i.e. S1 intensity is 10-99 pfu, S2 is 100-999 pfu, etc. Solar Cycle 23 featured six major storms of S4 intensity, although none has yet occurred in the current Solar Cycle 24.|
Notably, there is only a limited impact of this increased solar activity in the performance of the instruments or the quality of the science products at the levels of activity observed so far. Unlike many other astronomical satellites, Herschel has proved to be highly resistant to the effects of solar storms. Higher levels of proton flux increase the observed glitch rate and background offsets due to charge transfer from proton hits changing the effective detector biasing, most notably in PACS spectrometer observations. While chopped modes are relatively robust against such effects, the unchopped PACS spectroscopy mode is particularly vulnerable to possible degradation during solar proton storms.
|After a proton storm, any data that are potentially affected are carefully assessed in the Quality Control process, including an inspection by eye by one of the HSC instrument experts. At the time of writing, no observation has been failed in the Quality Control process so far in the mission. Any seriously compromised observation that is identified in the Quality Control process will be flagged and repeated automatically and the Principal Investigator of the programme alerted to the issue via the HSC Helpdesk.|
Plots giving the calibrated count rates in eight energy ranges from 12.6 to 166 MeV for 2010, 2011 and early 2012 are displayed in Figure 4.4 Figure 4.6 and Figure 4.7 respectively. The increase in activity during the mission is obvious; note how the vertical scale increases by three orders of magnitude between 2010 and 2012. Solar proton events are linked to long duration x-ray flares, almost always of M and X-class, which give Coronal Mass Ejections. For a solar proton storm to impact the Herschel spacecraft it must be generated either by a sunspot close to the centre of the Sun's disk, or by a sunspot that is magnetically connected to the Earth. Occasionally a solar proton event may even occur from a flare that is just over the solar limb if it is strongly magnetically connected to the Earth.
Figure 4.4. SREM calibrated count rates from the archive of calibrated SREM data http://proteus.space.noa.gr/~srem/herschel/ for the year 2010.
In Figure 4.8 the three years of data are presented on the same vertical scale for easier direct comparison and to make the variation in the cosmic ray flux - the base level for the plots - clearer. The most obvious feature is the progressive decline in the cosmic ray flux in the first half of 2010 as the heliosphere expanded outwards with increased solar activity and served to attenuate the flux arriving at L2, which reached record levels during the recent solar minimum, when the measured cosmic ray flux was over 20% higher than at any previous time since observations began in 1958. There is a suggestion of a small degree of time variability in the cosmic ray flux over scales of a few months, such as a further decrease in the first half of 2011; as the calibration of the SREM data is improved the reality of such possible variability will become clearer.
Figure 4.5. SREM calibrated count rates for the 2012 January 23 and January 28 solar proton storms. The arrival of the shock wave from the Coronal Mass Ejection can be seen as a sharp peak in the proton flux approximately 36 hours after the initiation of the January 23 event.
Solar proton events, as seen increasingly in the SREM data through 2011 and 2012, are soft, with the amplitude dropping rapidly to higher energies. However, events differ widely in energy spectrum and so may not be comparable. The reference energy for solar proton events is the 10 MeV flux (see: http://umbra.gsfc.nasa.giv/SEP/ for a listing of all solar proton events since 1976, with some useful background information) but, for example, although the events of 2012 January 23 (Figure 4.5 ) and 2012 March 7 were of similar amplitude at the reference energy of 10 MeV, the latter was approximately a factor of 6 larger at 166 MeV. For geoeffective Coronal Mass Ejections, in which the Coronal Mass Ejection is aimed at the Earth (this is usually the case when the sunspot is close to the centre of the solar disk), the peak 10 MeV flux is usually measured when the Coronal Mass Ejection shock wave hits, typically around 24-36 hours after the start of the event; this shock wave may have only a very low amplitude at energies above 100 MeV and occur when the flux of higher energy protons is already declining rapidly. We find no evidence that there is a correlation between solar proton storms and Single Event Upsets (SEUs) - bit flips in the memory - on board the spacecraft. All evidence suggests that SEUs are linked to impacts from high energy cosmic rays, well above the energies registered in solar proton storms.
Figure 4.6. SREM calibrated count rates from the archive of calibrated SREM data http://proteus.space.noa.gr/~srem/herschel/ for the year 2011.
Figure 4.7. SREM calibrated count rates from the archive of calibrated SREM data http://proteus.space.noa.gr/~srem/herschel/ for the year 2012.
Figure 4.8. SREM calibrated count rates from the archive of calibrated SREM data http://proteus.space.noa.gr/~srem/herschel/ for the year 2010-2012 on the same vertical scale. This representation shows the variation of the base level, that is, the cosmic ray element of the proton flux more clearly. In particular, there is a decrease in the cosmic ray flux in early 2010 and there may also be some low-level variations at other times.