2010-10-21 12:48:39

First Forbush decrease of the 24th Solar Activity Cycle registered by Aragats Neutron Monitors

After long absolutely calm period, Sun started to presentfirst signs of activity.  A shock detected by the Ace Spacecraft at 3August indicated the arrival of at least one CME (Coronal Mass Ejection) causedby the long duration C3 flare on August 1.

 

 Figure 1.Extreme ultraviolet snapshot from the Solar Dynamics Observatory (SDO) showsthe sun's northern hemisphere in mid-eruption at 1 August, 2010. Differentcolors in the image represent different gas temperatures ranging from ~1 to 2million degrees K. Credit: NASA/SDO

CME hit Earth's magnetic field on August 3rd at 17:40 UT. TheKp-index reached 6 (DST index -70nT) and this indicates a G2 Level GeomagneticStorm, lasted nearly 12 hours. Correspondingly Neutron monitors demonstratecoherent changes of the hourly count rates as seen in the Figure 2. Thestrength of the geomagnetic storm was not very large and  cosmic ray intensity depletion (so calledForbush decrease) do not enhance 2.5%; however very similar pattern of theintensity changes demonstrate by remote monitors do not leave any doubts thatthey are caused by the disturbances of the geomagnetic field due to arrivedhuge clouds of magnetized solar plasma. Other ASEC monitors as well as worldwidenetwork of SEVAN monitors did not register event due to higher cutoff rigidity.

  

 Figure 2.Geomagnetic storm index (planetary DST index) and count rates of 2 Aragats(ArNM and NANM)and Alma-Ati (AATB)Neutron Monitors