1. Forbush Decreases and Interplanetary Coronal Mass Ejections at Earth and Mars
Mark Lester1, Beatriz Sanchez-Cano1, Emma Thomas1, Adam Langeveld1, Jingnan Guo2, Hermann Opgenoorth3 and Olivier Witasse4
1 University of Leicester, UK
2 University of Kiel, Germany
3 IRF, Uppsala, Sweden
4 ESA, Noordwijk, Netherlands

2. Motivation
Observations of the Martian space plasma environment have often been limited due to the lack of solar wind information.
Some studies have used data at 1 AU as proxies for solar wind observations at Mars (e.g. Opgenoorth et al., 2013)
Studies have also used solar wind model predictions based on ENLIL
Limited to only part of Mars orbit, however, making it harder to utilise the whole data sets of some missions, e.g. Mars Express
Opgenoorth et al., 2013

3. Motivation Forbush decreases were first described in the 1930s
Current understanding is that these result from the passage of a CME past Earth (e.g. Richardson and Cane, 2011)
Recent studies have demonstrated the utility of data from MSL RAD to identify Forbush decreases (e.g. Witasse et al., 2017; Guo et al., 2017)
Richardson and Cane, 2011

4. Instrumentation and Study Interval
Earth
Surface – Bartols Neutron Monitors
Space – ACE Cosmic Ray Isotope Spectrometer (CRIS)
Mars
Surface – MSL RAD
Space – Mars Odyssey HEND (Boynton et al., 2004). We use the Outer Scintillator (an anti-coincidence detector) in channels 9-16
MAVEN data also where possible
Identified CME events from the LASCO CME data base ( 
We have also used the ENLIL simulations to provide a nominal prediction of the time of arrival of the event at Mars and Earth (
Focussed on the years 2012 – 2014 for this study.
Mars and Earth were reasonably well aligned for 2012 and 2014 while on opposite sides of the Sun for 2013
Note that CRIS Measures GCRs but Neutron Monitors and HEND measure secondary electrons
The High Energy Neutron Detector (HEND) [Boynton et al., 2004] on board the Mars Odyssey spacecraft allows the identification of GCRs and SEPs in orbit around Mars. The HEND instrument is composed of five separate sensors that provide measurements of neutrons in the energy range from 0.4 MeV up to 15 MeV [Zeitlin et al., 2010]. We use the Outer Scintillator (an anti-coincidence detector) in channels The detector provide information on minor to moderate solar particle events.
It's a proxy of GCR since it measures the secondary particles produced after a GCR hit the spacecraft, or it's back from the atmosphere of Mars after reflection at the surface of the planet.
I think these are the 2 main datasets that we are using.... if I'm forgetting anyone, please let me know.

5. Earth Orbit Mars Orbit 92 Day No 92 Day No 100 100 ACE CRIS HEND
ACE IMF
MAVEN IMF
Solar wind n
MEX Solar wind n
Solar wind v
MEX Solar wind v
Solar wind p
MEX Solar wind p 92
Day No 92
Day No
100
100

6. Super Posed Epoch Comparison of data for April 2014 event
Bartols
MSL
ACE
HEND

7. Some Statistics Earth Mars Both 2012 21 18 7 2013 11 16 1 2014 22 6
FD Period (Days)
Recovery Time (Days)
% Decrease
Earth Orbit
0.70 +/- 0.1
9.30 +/- 0.08
7.77
Earth Surface
0.60 +/- 0.1
/- 0.08
2.30
Mars Orbit
0.62 +/- 0.01
6.98 +/- 0.01
3.30
Mars Surface
0.66 +/- 0.01
/- 0.01
6.40
Using the observations of the CMEs together with predicted timing and observations
Occurrence of Forbush Decreases does not appear to be different at any point in the orbit
The one year planets have largest solar longitude separation see least co-incidence
Period of the actual Forbush decrease is similar at all locations
Recovery time and the % decrease do appear to differ.
These differences could be due to the different instrumentation.

8. Event showing no correspondance
Location of planets in heliosphere for the event on 27th July 2013
Mars and Earth are separated by about 135o solar longitude

9. HEND Data July 2013
HEND sees a rapid decrease of about 7.6% which is slightly higher than the average
The duration of the decrease is around 8 hours which is much less than the average (14 hours)
Recovery time is of order 6 days, which is typical of the average

10. MSL Data July 2013
MSL sees a reduction in dose rate of about 10% (higher) over the same duration as HEND
Recovery time is about the same

11. Earth-Mars Comparison July 2013 Event

12. ENLIL Simulation July 2013 Event
3 stills from the previous slide ENLIL simulation.
The CME was ejected on the 22nd and arrived at Mars on the 27th at ~8am
Matches very well with the HEND observations (only 2h difference)

13. Summary and Conclusions
Statistical investigation of the properties of Forbush decreases seen at 4 places in the heliosphere at L1, at Earth’s Surface, in Mars orbit and at Mars surface
Observations by HEND correspond well with those seen elsewhere in determining the onset of Forbush Decrease
HEND does suffer from the effect of SEP events, however, which can cause a reduction in the number of events that we can use
Conclude that this data set could be used as a proxy for the investigation of ICME effects at Mars