Environmental conditions during the charging anomaly of the two geosynchronous satellites reported: TELSTAR 401 and Galaxy 15 Elena Saiz, A. Guerrero, C. Cid, J. Palacios, Y. Cerrato Space Weather Group University of Alcalá (Spain) elena.saiz@uah.es

Introduction In a technologically developed society, the good state of its satellites in the near-Earth space is crucial During substorms, low-energy (< 40 keV) and higher energy (> 100 keV) electrons can be injected into the near‐Earth region. They can deposit in the spacecraft surfaces or penetrate structures causing surface charging and internal charging respectively, posing a risk of electrostatic discharge (ESD) The two GEO satellite anomalies reported as related to ESD during 1997–2010 spacecraft anomaly date UT MLT References Telstar 401 1997, Jan. 11 11:15 4.8 Choi et al. (2011) Galaxy 15 2010, Apr. 5 09:48 0.9 Ferguson et al. (2011) For both events we compare: magnetospheric B observed by GOES satellites ∆H variations at mid- and low-latitude stations to check whether similar magnetospheric and/or interplanetary conditions could increase risk of ESD Relative location of spacecraft in GSM X-Y plane (in RE) for the time of each anomaly

Ground magnetic disturbances Day-night asymmetry (H < 0-H > 0) at mid/low latitude stations: a distinct signature of magnetic disturbance caused by FACs S → shock Di → dipolariz. ∆H > 0 ∆H > 0 ∆H < 0 ∆H < 0 ∆H > 0 ∆H > 0 ∆H < 0 ∆H < 0

Relevant small-scale IP conditions Turning northward Bz, pressure changes, By changes,... all of them identified as potential triggers of substorm

But how extraordinary was it in its operational life? TELSTAR 401 GALAXY 15 But how extraordinary was it in its operational life? Only a previous event was recorded with high IP parameters (Ey, Pdyn, dBy/dt) simultaneously. Fortunately, at that time they were in an “adequate sector”    Comparison analysis and Conclusions Magnetosph The anomalies took place during the expansion phase of a triggered substorm with two dipolarizations close in time Strong asymmetry in B observed by G8, G9 at Telstar event (and subsequent asymmetry in the particle fluxes) ULF pulsations observed in Telstar event  Ground Magnetic disturbances of FACs were observed at mid/low latitude stations. This distinct signature is revealed as an unmistakable indicator of substorms instead of the classic index AL. This is relevant when observations from space are limited The anomalies took place during a moderate magnetic storm (Galaxy 15 during the main phase and Telstar 401 in the recovery phase), but SYM-H, Dst or Kp did not result good proxies as they miss short-time and local magnetic disturbances as FACs are  IP The substorm triggers were favored by the interaction ICME-HSS Sat. The anomalies occurred at the worst scenario in the operational life of the satellites. Both were in the wrong place (0-6 LT sector) at a critical time interval (elevated interplanetary parameters simultaneously), supporting the location as a key issue for ESD