[1] Forbush decrease (FD) events are supposed to happen simultaneously over the globe of the Earth. However, there have been several reports on nonsimultaneous FD events. We investigate for the first time the properties of nonsimultaneous FD events and the solar wind conditions causing such events in detail in order to determine what solar wind conditions lead to global simultaneity of FD events. We examined the hourly data of the Oulu Neutron Monitor (NM) station from 1998 to 2002. We have selected 49 FD events that have greater than 3.5% intensity reductions. Global simultaneity was determined by comparing the time profiles of these FD events with those recorded by other NM stations at Inuvik and Magadan. These three NM stations are located at close magnetic latitude (cutoff rigidity) but fairly evenly spaced in longitudes. The solar wind parameters driving FD event main phase are studied. Most FD event onsets (37 out of 49) are observed simultaneously by each NM station in universal time (UT) regardless of the location of the station, whereas some other FD events are not simultaneously detected but are at similar local times (LT). The stronger FD events tend to be simultaneous events, but the weaker FD events are nonsimultaneous. The simultaneous FD events are caused by the solar wind of higher speed and stronger IMF, whereas the nonsimultaneous events are driven by relatively lower speed and weaker IMF solar wind. The nonsimultaneous FD event may occur only if the main phase of the FD is superposed in phase with the declining phase of diurnal variation. Our interpretation is that the simultaneous FD events might occur when the high-speed strong magnetic barrier (interplanetary shock sheath and magnetic cloud) overtakes the Earth, whereas the nonsimultaneous FD events may occur only when the slow-moving weak magnetic barrier passes by on the duskside of the Earth. The global simultaneity of FD events depends on speed and IMF strength of solar wind overtaking Earth's magnetosphere and its propagation direction. This model of FD simultaneity can be tested by the STEREO mission.

Forecasting space weather more accurately from solar observations requires an understanding of the variations in physical properties of interplanetary (IP) shocks as solar activity changes. We examined the characteristics (occurrence rate, physical parameters, and types of shock driver) of IP shocks. During the period of 1995 -2001, a total of 249 forward IP shocks were observed. In calculating the shock parameters, we used the solar wind data from Wind at the solar minimum period (1995 -1997) and from ACE since 1998 including the solar maximum period (1999 -2001). Most of IP shocks (68%) are concentrated in the solar maximum period. The values of physical quantities of IP shocks, such as the shock speed, the sonic Mach number, and the ratio of plasma density compression, are larger at solar maximum than at solar minimum. However, the ratio of IMF compression is larger at solar minimum. The IP shock drivers are classified into four groups: magnetic clouds (MCs), ejecta, high speed streams (HSSs), and unidentified drivers. The MC is the most dominant and strong shock driver and 150 out of total 249 IP shocks are driven by MCs. The MC is a principal and very effective shock driver not only at solar maximum but also at solar minimum, in contrast to results from previous studies, where the HSS is considered as the dominant IP shock driver.

[1] In this study we have made a forecast evaluation of geoeffective coronal mass ejections (CMEs) by using frontside halo CMEs and the magnetospheric ring current index, Dst. This is the first time, to our knowledge, that an attempt has been made to construct contingency tables depending on the geoeffectiveness criteria as well as to estimate the probability of CME geoeffectiveness depending on CME location and/or speed. For this, we consider 7742 CMEs observed by SOHO/LASCO and select 305 frontside halo CMEs with their locational information from 1997 to 2003 using SOHO/ EIT images and GOES data. To select CME-geomagnetic storm (Dst < À50 nT) pairs, we adopt a CME propagation model for estimating the arrival time of each CME at the Earth and then choose the nearest Dst minimum value within the window of ±24 hours. For forecast evaluation, we present contingency tables to estimate statistical parameters such as probability of detection yes (PODy) and false alarm ratio (FAR). We examine the probabilities of CME geoeffectiveness according to their locations, speeds, and their combination. From these studies, we find that (1) the total probability of geoeffectiveness for frontside halo CMEs is 40% (121/305); (2) PODys for the location (L < j50°j) and the speed (>400 km s À1 ) are estimated to be larger than 80% but their FARs are about 60%; (3) the most probable areas (or coverage combinations) whose geoeffectiveness fraction is larger than the mean probability ($40%), are 0°< L < +30°for slower speed CMEs ( 800 km s À1 ), and À30°< L < +60°for faster CMEs (>800 km s À1 ); (4) when the most probable area is adopted as the new criteria, the PODy becomes slightly lower, but all other statistical parameters such as FAR and bias are significantly improved. Our results can give us some criteria to select geoeffective CMEs with the probability of geoeffectiveness depending on the location, speed, and their combination.

Frontside halo coronal mass ejections (CMEs) are generally considered as potential candidates for producing geomagnetic storms, but there was no definite way to predict whether they will hit the Earth or not. Recently Moon et al. suggested that the degree of CME asymmetries, as defined by the ratio of the shortest to the longest distances of the CME front measured from the solar center, be used as a parameter for predicting their geoeffectiveness. They called this quantity a direction parameter, D, as it suggests how much CME propagation is directed to Earth, and examined its forecasting capability using 12 fast halo CMEs. In this paper, we extend this test by using a much larger database (486 frontside halo CMEs from 1997 to 2003) and more robust statistical tools (contingency table and statistical parameters). We compared the forecast capability of this direction parameter to those of other CME parameters, such as location and speed. We found the following results: (1) The CMEs with large direction parameters (D ! 0:4) are highly associated with geomagnetic storms. (2) If the direction parameter increases from 0.4 to 1.0, the geoeffective probability rises from 52% to 84%. (3) All CMEs associated with strong geomagnetic storms ( Dst À200 nT) are found to have large direction parameters (D ! 0:6). (4) CMEs causing strong geomagnetic storms (Dst À100 nT), in spite of their northward magnetic field, have large direction parameters (D ! 0:6). (5) Forecasting capability improves when statistical parameters (e.g., ''probability of detection -yes'' and ''critical success index'') are employed, in comparison with the forecast solely based on the location and speed of CMEs. These results indicate that the CME direction parameter can be an important indicator for forecasting CME geoeffectiveness. Subject headingg s: solar-terrestrial relations -Sun: coronal mass ejections (CMEs)

[1] Neutron monitors have recorded the flux of high-energy Galactic cosmic rays for more than half a century. During the recent, prolonged, deep minimum in solar activity, many sources indicate that modulated Galactic cosmic rays have attained new Space Age highs. However, reported neutron monitor rates are ambiguous; some record new highs while others do not. This work examines the record of 15 long-running neutron monitors to evaluate cosmic ray fluxes during the recent extraordinary solar minimum in a long-term context. We show that ground-level neutron rates did reach a historic high during the recent solar minimum, and we present a new analysis of the cosmic ray energy spectrum in the year 2009 versus year 1987. To do this, we define a reference as the average of eight high-latitude neutron monitors, four in the Northern Hemisphere (Apatity, Inuvik, Oulu, Thule) and four in the Southern Hemisphere (Kerguelen, McMurdo, Sanae, Terre Adelie). Most stations display changes in sensitivity, which we characterize by a simple linear trend. After correcting for the change in sensitivity, a consistent picture emerges. With our correction, all stations considered display new highs at the recent solar minimum, approximately 3% above the previous record high. These increases are shown to be consistent with spacecraft observations.