3:45 PM - 4:00 PM
[PEM15-02] Relation between interplanetary scintillation indices and space weather
Keywords:solar wind, interplanetary scintillation, space weather
Interplanetary scintillation (IPS) observed for compact radio sources is a scattering phenomenon caused by density irregularities (ΔNe) in the solar wind. The intensity of IPS, which is weighted integration of ΔNe along the line of sight (los), varies intricately depending on the separation distance of the los from the sun and the structure of the solar wind. Furthermore, it also depends on the apparent size of radio source. Therefore, the IPS intensities for different radio sources cannot be readily compared to each other. The g-value was introduced in order to solve this problem (Gapper et al., 1982). The g-value is defined as a ratio of the observed IPS intensity to the value given by the model curve of the solar elongation dependence of IPS: the g-value is around unity for the quiet solar wind, and it becomes g>1 (<1), when the los intersects a high (low)-density region. Since the effect of the apparent source is included in the model curve, the g-value enables comparison of data for different sources. Then, a sky projection map of ΔNe (g-map) can be produced from g-values observed for many sources in a day. The g-value data have been collected every day from IPS observations at 327 MHz at ISEE of Nagoya University. The 3-D structure and propagation property of interplanetary disturbances associated with CMEs have been revealed form analysis of ISEE g-value data (Tokumaru et al., 2000, 2003, 2005, 2007; Iju et al., 2013, 2014).
The g-map is useful for gaining the whole view of the inner heliosphere. However, it contains too much information reflecting the solar wind for non-experts to interpret easily. In this study, we derived the indices from ISEE g-value data on a daily basis, and investigated relation between them and space weather. The attempt to calculate the index from g-value data was first made using Cambridge IPS observations at 81 MHz (Harrison et al., 1992; Hapgood & Harrison, 1994; Lucek et al., 1995, 1996a, 1996b). In their studies, the indices such as I35, Ihi, Gave were defined and compared with geomagnetic data. Those indices were found to be useful to some extent for space weather predictions, however they were significantly affected by the ionosphere because of the low observation frequency. Since the observation frequency of ISEE is higher than that of Cambridge, our g-value data is less affected by the ionosphere. Therefore, we expect that relation between the IPS index and the space weather becomes more clearly from this study. In this study, we derived I35, Ihi, Gave from ISEE g-value data between 1997 and 2019, and compared them with the solar wind (density and speed) at the earth, and Dst index. The daily mean values of the solar wind data and the Dst index were derived to compare with the IPS indices. Note that the Kiso and Toyokawa antennas were employed for 1997-2009 (Cycle 23) and 2008-2019 (Cycle 24), respectively for g-value measurements.
We found that the positive correlation between the solar wind density and the IPS indices showed a peak at a time difference of 0 day, and that positive correlations between the solar wind speed and the IPS indices occurred after +1 day, lasting for a few days. This tendency is observed for both Cycles 23 and 24. This result suggests that increase in the IPS index corresponds to the compression region at the boundary between fast and slow winds. We also found that a negative correlation between Dst and IPS indices occurred after +1 day for Cycle 23. However, this tendency was not found for Cycle 24. These results are not mutually exclusive because the IMF plays an important role in development of Dst.
The g-map is useful for gaining the whole view of the inner heliosphere. However, it contains too much information reflecting the solar wind for non-experts to interpret easily. In this study, we derived the indices from ISEE g-value data on a daily basis, and investigated relation between them and space weather. The attempt to calculate the index from g-value data was first made using Cambridge IPS observations at 81 MHz (Harrison et al., 1992; Hapgood & Harrison, 1994; Lucek et al., 1995, 1996a, 1996b). In their studies, the indices such as I35, Ihi, Gave were defined and compared with geomagnetic data. Those indices were found to be useful to some extent for space weather predictions, however they were significantly affected by the ionosphere because of the low observation frequency. Since the observation frequency of ISEE is higher than that of Cambridge, our g-value data is less affected by the ionosphere. Therefore, we expect that relation between the IPS index and the space weather becomes more clearly from this study. In this study, we derived I35, Ihi, Gave from ISEE g-value data between 1997 and 2019, and compared them with the solar wind (density and speed) at the earth, and Dst index. The daily mean values of the solar wind data and the Dst index were derived to compare with the IPS indices. Note that the Kiso and Toyokawa antennas were employed for 1997-2009 (Cycle 23) and 2008-2019 (Cycle 24), respectively for g-value measurements.
We found that the positive correlation between the solar wind density and the IPS indices showed a peak at a time difference of 0 day, and that positive correlations between the solar wind speed and the IPS indices occurred after +1 day, lasting for a few days. This tendency is observed for both Cycles 23 and 24. This result suggests that increase in the IPS index corresponds to the compression region at the boundary between fast and slow winds. We also found that a negative correlation between Dst and IPS indices occurred after +1 day for Cycle 23. However, this tendency was not found for Cycle 24. These results are not mutually exclusive because the IMF plays an important role in development of Dst.