[1] Ground-level enhancements (GLEs) are sudden, sharp, and short-lived increases in cosmic ray intensities registered by neutron monitors. These enhancements are known to take place during powerful solar eruptions. In the present investigation, the cosmic ray intensities registered by the Oulu neutron monitor have been studied for the period between January 1979 and July 2009. Over this span of time, increase rates of 32 GLEs have been deduced. In addition, we have studied characteristics of the 32 event-associated solar flares, coronal mass ejections (CMEs), and solar energetic particle (SEP) fluxes. We found that all of the 32 GLEs were associated with solar flares, CMEs, and SEP fluxes. Approximately 82% of the events were associated with X-class flares. Most of the flares that were associated with GLEs of increase rates >10% originated from the active regions located on the southwest hemisphere of the Sun. The average speed (1726.17 km/s) of GLE-associated CMEs was much faster than the average speed (423.39 km/s) of non-GLE-associated CMEs. It also became evident that ∼67% GLEs were associated with very fast (>1500 km/s) CMEs. Although a GLE event is often associated with a fast CME, this alone does not necessarily cause the enhancement. Solar flares with strong optical signatures may sometimes cause GLE. High SEP fluxes often seem to be responsible for causing GLEs as the correlation with SEP fluxes implies.

The flux rate of cosmic rays incident on the Earth's upper atmosphere is modulated by the solar wind and the Earth's magnetic field. The amount of solar wind is not constant due to changes in solar activity in each solar cycle, and hence the level of cosmic ray modulation varies with solar activity. In this context, we have investigated the variability and the relationship of cosmic ray intensity with solar, interplanetary, and geophysical parameters from January 1982 through December 2008. Simultaneous observations have been made to quantify the exact relationship between the cosmic ray intensity and those parameters during the solar maxima and minima, respectively. It is found that the stronger the interplanetary magnetic field, solar wind plasma velocity, and solar wind plasma temperature, the weaker the cosmic ray intensity. Hence, the lowest cosmic ray intensity has good correlations with simultaneous solar parameters, while the highest cosmic ray intensity does not. Our results show that higher solar activity is responsible for a higher geomagnetic effect and vice versa.

We examine the atmospheric drag on the low earth-orbiting satellite, KOMPSAT-1 in a sun-synchronous orbit at ∼685-km altitude starting in 1999, during a 3-month (October-December) period in 2003. This 3-month interval includes the October 29-30 and November 20 magnetic superstorms and weak to moderate storms. We observed that the daily KOMPSAT-1 drag acceleration transiently responses to transient storm-time disturbances. That is, there is an one-to-one correspondence between the KOMPSAT-1 drag accelerations and the storm events. We find that the drag acceleration correlates strongly with the level of geomagnetic disturbance. This indicates that the trajectory of KOMPSAT-1 is significantly perturbed during extremely disturbed intervals because of atmospheric density increase. The main contributor to the density increase is Joule heating associated with the geomagnetic activity rather than the solar EUV radiation, as reported by previous studies. We suggest that understanding how the upper atmosphere responses to the geomagnetic-associated heating is important to predict space weather impacts on low earth-orbiting satellites.

[1] We made an effort to understand the associations and relationships between ground level enhancement (GLE) events and solar flares for the time period of 1986-2006. Our results show that, on average, the GLE event-associated solar flare (∼0.2 × 10 −4 W/m 2 ) is much stronger than the non-GLE-associated solar flare (∼0.3 × 10 −5 W/m 2 ). The findings have also been supported by the solar flare indices that, on average, the GLE event-associated solar flare index (∼35.01) is much higher than the non-GLE-associated solar flare index (∼4.88). However, this association does not seem to precisely imply that GLEs can occur because of a solar flare, so we examined cross correlations between GLE events and simultaneous solar flares. We found that most (∼78%) of the highest correlations (r > 0.8) took place during an X class flare. There is no clear indication that the more the time lag, the less or more is the correlation or vice versa. Overall, 50% of the high correlations took place at higher time lag (≥65 min), and ∼36% of the high correlations took place at lower time lag (≤40 min), while the rest (∼14%) of the correlations were abruptly high and low at medium time lag (>40 and <65 min). On the basis of the results of cross correlations, we suggest that the intensive portions of solar flares should be responsible for causing GLEs and that the direct proportionality of the time-integrated intensive portion of a flare with the impulsive phase of a GLE event seems to be the main property for comprehending the mechanism.