Mars Express and Venus Express multi-point observations of geoeffective solar flare events in December 2006

In December 2006, a single active region produeed a series 01 proton solar tlares, with X-ray class up to X9.0 Icvcl, starting on 5 Deeember 2006 at 10:35 UT. A feature 01 this X9.0 tlare is that assoeiated McV parti cles were observed at Venus and Mars by Venus Express (VEX) and Mars Express (MEX), which were -800 and -1250 cast ofthe tlare site, respectively, in addition to the Earth, whieh was -790 west 01 the tlare site. On 5 December 2006. the plasma instruments ASPERA-3 and ASPERA-4 on board MEX and VEX deteeted a large enhancement in their respective background count levels. This is a typieal signature 01 solal energetic partielc (SEP) events, i.e., intensive McV particle tluxes. The timings 01 these enhancements were consistent with the estimated ficld-aligned travcl time 01 particlcs assoeiated with the X9.0 tlare that followed the Parker spirai to reach Venus and Mars. Coronai mass ejection (CM E) signatures that might be related to the proton Ilare were twice identified at Venus within <43 hours and <67 hours after the tlare. Although these CMEs did not necessarily originate lrom the X9.0 Ilare on 5 Deeember 2006, they most likcly originated liom the same aetive region because these eharacteristies are very similar to Ilare assoeiated CMEs observed at the Earth. These observations indicate that CME and tlare activities on the invisible side 01 the Sun may affeet terrestrial spaee weather as a result 01 traveling more than 900 in both azimuthal directions in the hcliosphere. Wc would also like to emphasize that during the SEP activity, Mars Express data indicate an approximately one-order of magnitude enhancement in the hcavy ion outllow tlux tiom the Martian atmosphcre. This is the first observation of the inerease 01 escaping ion llux lrom Martian atmosphere during an intensive SEP event. This suggests that the solar EUV llux levcls signifieantly affect the atmospheric loss lrom unmagnctized planets.