Interhourly variability index of geomagnetic activity and its use in deriving the long-term variation of solar wind speed
Interhourly variability index of geomagnetic activity and its use in deriving the long-term variation of solar wind speed
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地磁活动的时际变化指数及其在推导太阳风速长期变化中的应用
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发表时间：
2007
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通讯作者：
E. Cliver
中科院分区：
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作者：
L. Svalgaard;E. Cliver
[1]我们描述了地磁活动的小时间变率(IHV)指数的详细推导。给定地磁元素的IHV指数机械地从小时值或平均值得出，即在以当地午夜为中心的7小时间隔内相邻小时之间的无符号差值的总和。该指数是针对地球上六个经度扇区内的两个半球的站点分别计算的，仅使用当地夜间时间。它的目的是作为一种长期指数，现有数据允许将该指数追溯到19世纪。在Bartels旋转27天的时间尺度上，校正地磁纬度小于55°的台站的IHV平均值与中纬度范围指数有很强的相关性(自1959年以来AM指数的R2=0.96；1980年以来AA指数的R2=0.95)。我们发现，1957年以前观测到的AA的年平均值与根据1980年至2004年的回归常数根据IHV计算的值相比，∼3 nT太小了。我们将这一差异解读为1957年前AA指数校准有误的迹象。追溯到1932年，观测到的AP值与类似计算的AP值之间没有系统性差异。IHV的Bartels旋转平均值也与太阳风参数有很强的相关性(BV2的R2=0.79)。在一年的时间尺度上，结合IHV指数(给出BV2，R2=0.93)和最近发展的日间变化(IDV)指数(给出行星际磁场强度B，R2=0.74)，可以确定从1890年到现在的太阳风速V。在∼的120年系列中，年平均太阳风速从1902年的低(推断)303公里/S到2003年的高(观测)545公里/S。用分别由IDV和IHV得到的B和V计算的乘积BV的年值与从1926年以来极冠水平分量的日变化幅度得出的(完全独立的)BV值(以及更早的零星变化)定量地一致。
[1] We describe the detailed derivation of the interhourly variability (IHV) index of geomagnetic activity. The IHV index for a given geomagnetic element is mechanically derived from hourly values or means as the sum of the unsigned differences between adjacent hours over a 7-hour interval centered on local midnight. The index is derived separately for stations in both hemispheres within six longitude sectors spanning the Earth using only local night hours. It is intended as a long-term index and available data allows derivation of the index back well into the 19th century. On a timescale of a 27-day Bartels rotation, IHV averages for stations with corrected geomagnetic latitude less than 55° are strongly correlated with midlatitude range indices (R2 = 0.96 for the am index since 1959; R2 = 0.95 for the aa index since 1980). We find that observed yearly averages of aa before the year 1957 are ∼3 nT too small compared to values calculated from IHV using the regression constants based on 1980–2004. We interpret this discrepancy as an indication that the calibration of the aa index is in error before 1957. There is no systematic discrepancy between observed and similarly calculated ap values back to 1932. Bartels rotation averages of IHV are also strongly correlated with solar wind parameters (R2 = 0.79 with BV2). On a timescale of a year combining the IHV index (giving BV2 with R2 = 0.93) and the recently developed interdiurnal variability (IDV) index (giving interplanetary magnetic field magnitude, B, with R2 = 0.74) allows determination of solar wind speed, V, from 1890 to present. Over the ∼120-year series, the yearly mean solar wind speed varied from a low (inferred) of 303 km/s in 1902 to a high (observed) value of 545 km/s in 2003. The calculated yearly values of the product BV using B and V separately derived from IDV and IHV agree quantitatively with (completely independent) BV values derived from the amplitude of the diurnal variation of the horizontal component in the polar caps since 1926 (and sporadically further back).