Measurements of Forbush decreases at Mars: comparison of measurement by MAVEN in orbit and by MSL on ground Jingnan Guo1, Robert Lillis2, Robert F. Wimmer-Sch eingruber!, "ary #eitlin$, %atric& Simonson!,', (li Rahmati2, (rik %osner), (thanasios %apaioannou*, +iklas Lundt!, "hristina ,. Lee2, -a.in Larson2, /asper 0alekas1, -onald M. 0assler2, 3ent 4hresmann2, %atric& -unn2, and Stephan 35ttcher! 6!) 8ni.ersity of 9iel, Kiel, :ermany (2) SSL, 8ni.ersity of "alifornia, 3er&eley "(, 8S( 6$) Leidos, +(SA /ohnson Space center, ;X, 8SA 6'7 "ollege of "harleston, South "arolina, 8SA 6)) +(SA 0ead=uarter, Washington -", 8SA 6*7+ational ,bser.atory of (thens, :reece 61) South est Research >nstitute, 3oulder, ",, 8S( 62) 8ni.ersity of >o a, >o a "ity, >o a, 8SA November 2017, ESWW, Ostend, elgium ref# Guo et a! 2017 A$A %submitted& 1 GCR and Forbush decreases measured at Mars ● ,n the surface of Mars, the Radiation (ssessment -etector 6'A(7, on board Mars Science Laboratory?s 6MSL7 "uriosity ro.er, has been measuring ground le.el particle flu@es along ith the radiation dose rate at Mars surface since August 2012. ● Similar to neutron monitors at 4arth, R(- sees many )orbus* de+reases %)(s& in the galactic cosmic ray 6:CR7 induced surface flu@es and dose rates. ;hese F-s are associated ith coronal mass eAections 6"M4s7 andBor streamBcorotating interaction regions 6SIRsBCIRs7. ● Orbiting abo.e the Martian atmosphere, the Mars (tmosphere and Colatile 4.olutio+ 6MAVEN7 spacecraft has also been monitoring space eather conditions at Mars since September 2D!'. ;he penetrating particle flu@ channels in the Solar 4nergetic %article 6SE,7 instrument onboard M(C4+ can also be employed to detect F-s. ● For the first time, e study the statisti+s and -ro-erties of a !ist o" )(s obser.ed in-situ at Mars, seen both on the surface by MSLBR(- and in orbit detected by the M(C4+BSE% instrument. SEPs > 100 MeV Mars ● MAVEN/SEP in orbit Forbush decrease at Mars upon the arrival of an ICME ● MSL/RAD on ground 2 ref# Guo et a! 2017 A$A %submitted& >n total e made a list of !21 Forbush decreases related to "M4 and ">R encountering Mars during 2014 ,ct E 2016 Sep Such a list of FDs can be used for studying ICME propagation and SIR evolution through the inner heliosphere. MAVEN/SEP penetrating particle: protons > 100 MeV & e- > 600 keV MSL/RAD dose rate from primary GCRs >~ 100 MeV/nuc 3 ref# Guo et a! 2017 A$A %submitted& The fitted correlation between FD drop percentage at MSL and that at MAVEN is about 1.33 ± 0.14, i.e., an FD event seen on the surface of Mars by MSL with 10% drop magnitude corresponds to about 13.3 % drop magnitude of GCR protons with energies ≥ 100 MeV outside Mars’ atmosphere. The systematic difference between the magnitudes of the FDs within and outside the Martian atmosphere may be mostly attributed to the energy-dependent modulation of the GCR particles by both the heliospheric magnetic fields and the Martian atmosphere. 4 ref: :uo et al 2D!1 (F( 6submitted7 Ex-!anation o" the s/stemati+ di""eren+e between the am-!itude o" an FD in s-a+e and on the sur"a+e o" Mars (!ater is sma!!er&# (s the atmos-*ere slo s down particles, it ’shifts? the response weighting of the original G"Rs from lo -energy :"Rs to middle- and high-energy G"RsG meantime the *e!ios-*eri+ modu!ation of G"Rs is energ/-de-endent with lo -energy particles more deflected and their fluxes more suppressed by the increased magnetic fieldsG as a result, the Modulation of GCRs in space )(s on t*e sur"a+e o" Mars are a better pro@y of the modulation of *ig*er-energ/ G2's hich are less depressed by the enhanced heliospheric activities and therefore the magnitude of the same FD on the Martian sur"a+e is sma!!er than that outside t*e Martian atmos-*ere3 Modulation of GCRs on Mars 5 ref: :uo 2D!1 /:R-planet F :uo et al 2D!1 (F( 6submitted7.