D. Martini

Did Open Solar Magnetic Field Increase During The Last 100 Years? A Reanalysis of Geomagnetic Activity

Long-term geomagnetic activity presented by the aa index has been used to show that the heliospheric magnetic field has more than doubled during the last 100 years. However, serious concern has been raised on the long-term consistency of the aa index and on the centennial rise of the solar magnetic field. Here we reanalyze geomagnetic activity during the last 100 years by calculating the recently suggested IHV (Inter-Hour Variability) index as a measure of local geomagnetic activity for seven stations. We find that local geomagnetic activity at all stations follows the same qualitative long-term pattern: an increase from early 1900 to 1960, a dramatic dropout in 1960s and a (mostly weaker) increase thereafter. Moreover, at all stations, the activity at the end of the 20th century has a higher average level than at the beginning of the century. This agrees with the result based on the aa index that global geomagnetic activity, and thereby, the open solar magnetic field has indeed increased during the last 100 years. However, quantitatively, the estimated centennial increase varies greatly from one station to another. We find that the relative increase is higher at the high-latitude stations and lower at the low- and mid-latitude stations. These differences may indicate that the fraction of solar wind disturbances leading to only moderate geomagnetic activity has increased during the studied time interval. We also show that the IHV index needs to be corrected for the long-term change of the daily curve, and calculate the corrected IHV values. Most dramatically, we find the centennial increase in global geomagnetic activity was considerably smaller, only about one half of that depicted by the aa index.

Centennial increase in geomagnetic activity: Latitudinal differences and global estimates

[1] We study here the centennial change in geomagnetic activity using the newly proposed Inter-Hour Variability (IHV) index. We correct the earlier estimates of the centennial increase by taking into account the effect of the change of the sampling of the magnetic field from one sample per hour to hourly means in the first years of the previous century. Since the IHV index is a variability index, the larger variability in the case of hourly sampling leads, without due correction, to excessively large values in the beginning of the century and an underestimated centennial increase. We discuss two ways to extract the necessary sampling calibration factors and show that they agree very well with each other. The effect of calibration is especially large at the midlatitude Cheltenham/Fredricksburg (CLH/FRD) station where the centennial increase changes from only 6% to 24% caused by calibration. Sampling calibration also leads to a larger centennial increase of global geomagnetic activity based on the IHV index. The results verify a significant centennial increase in global geomagnetic activity, in a qualitative agreement with the aa index, although a quantitative comparison is not warranted. We also find that the centennial increase has a rather strong and curious latitudinal dependence. It is largest at high latitudes. Quite unexpectedly, it is larger at low latitudes than at midlatitudes. These new findings indicate interesting long-term changes in near-Earth space. We also discuss possible internal and external causes for these observed differences. The centennial change of geomagnetic activity may be partly affected by changes in external conditions, partly by the secular decrease of the Earth’s magnetic moment whose effect in near-Earth space may be larger than estimated so far. Citation: Mursula, K., and D. Martini (2006), Centennial increase in geomagnetic activity: Latitudinal differences and global estimates, J. Geophys. Res., 111, A08209, doi:10.1029/2005JA011549.

On the centennial trend estimates of geomagnetic activity indices

[1] In this paper we reanalyze the centennial trend estimates of a number of selected geomagnetic indices. We show that the peculiar latitudinal ordering of century long trends is mainly an artifact related to different scales. Using a scaling method that takes into account the statistical dispersion of the variables alleviates most of the discrepancies associated with latitude dependence. We demonstrate that geomagnetic activity had a roughly homogenous increase over the Northern Hemisphere during 1901–2000 of about 23%, as registered by digital measures. On the other hand, midlatitude data are shown to possess a distinct disproportion dominantly around solar cycle 15. We also demonstrate that analog indices tend to consistently depict lower activity levels in the early decades, thus considerably larger overall increases of about 32% during the twentieth century. This is likely due to the typically conservative approach in quantifying irregular activity (the K value) in those early years with a large number of quiet days. The recently corrected aa index shows an increase on par with similarly analog but local measures at Niemegk and Sodankyla stations. These results show that after its vital calibration, the widely used aa index does quantify reasonably well the global centennial increase in geomagnetic activity.

Long-term decrease in the response of midlatitude stations to high-speed solar wind streams in 1914–2000

We investigate geomagnetic activity at two high-latitude, two midlatitude, and two low-latitude stations of the northern hemisphere and find that the increasing trend of geomagnetic activity in 1914–2000 is considerably lower at the two midlatitude stations (Niemegk and Fredericksburg) than at low- or high-latitude stations. As Niemegk occupies a specific position among geomagnetic stations, serving as the standard station for the K/Ak index derivation, it is crucial to understand the origin and long-term characteristics of this difference. We show here that geomagnetic activity at the studied midlatitude stations is relatively stronger in the declining phases of the 1 and 2 first solar cycles (15 and 16) than elsewhere, leading to the smaller long-term trend. We also find that the latitudinal differences in the trends are strongly dependent on local time, being considerably larger in the dawn sector. These differences can be explained by the relatively stronger contribution of high-speed streams to geomagnetic activity at the particular range of midlatitudes, compared to the stations at lower and higher latitudes.

Services for Space Mission support within the ESA Space Situational Awareness Space Weather Service Network.

Spacecraft operations are by nature complex and every satellites operational environment poses a range of potential risks, often a unique combination for a given orbit. The implications of interruptions of operations, data transfer and service provision, are serious, both in terms of cost and capability, thus it is imperative to mitigate against all operational risks to the fullest extent possible. In the frame of its Space Situational Awareness (SSA) programme, the European Space Agency (ESA) is establishing a Space Weather Service Network to support end-users, in a wide range of affected sectors, in mitigating the effects of space weather on their systems, reducing costs and improving reliability. This service network is currently in a test and validation phase and encourages user engagement and feedback. The network is organised around five Expert Service Centres (ESCs) focusing on Solar Weather, Heliospheric Weather, Space Radiation Environment, Ionospheric Weather and Geomagnetic Conditions. Each ESC is connecting different expert groups, federating their space weather products, and ensuring the quality and consistency of the provided information. The service network also includes a central Data Centre and the SSA Space Weather Coordination Centre (SSCC). In this presentation we give an overview of the current status of the network (http://swe.ssa.esa.int/), the targeted end-user groups and Expert Service Centres with a focus on the space community.

Services for Spacecraft Operations support within the ESA Space Situational Awareness Space Weather Service Network

Spacecraft operations are by nature complex and every satellites operational environment poses a range of potential risks, often a unique combination for a given orbit. The implications of interruptions of operations, data transfer and service provision, are serious, both in terms of cost and capability, thus it is imperative to mitigate against all operational risks to the fullest extent possible. In the frame of its Space Situational Awareness (SSA) programme, the European Space Agency (ESA) is establishing a Space Weather Service Network to support end-users, in a wide range of affected sectors, in mitigating the effects of space weather on their systems, reducing costs and improving reliability. This service network is currently in a test and validation phase and encourages user engagement and feedback. The network is organised around five Expert Service Centres (ESCs) focusing on Solar Weather, Heliospheric Weather, Space Radiation Environment, Ionospheric Weather and Geomagnetic Conditions. Each ESC is connecting different expert groups, federating their space weather products, and ensuring the quality and consistency of the provided information. The service network also includes a central Data Centre and the SSA Space Weather Coordination Centre (SSCC). In this presentation we give an overview of the current status of the network (http://swe.ssa.esa.int/), the targeted end-user groups and Expert Service Centres with a focus on the space community. Keywords—space weather, space situational awareness, service network

Revisiting geomagnetic activity at auroral latitudes: No need for regular quiet curve removal for geomagnetic activity indices based on hourly data

The main objective of our study is to determine if the regular quiet daily curve (QDC) subtraction is a necessary procedure in quantifying the irregular geomagnetic variations at auroral latitudes. We define the hourly ΔH index, the absolute hour-to-hour deviation in nanotesla of the hourly geomagnetic horizontal component, which assigns each sample to sample deviation as geomagnetic activity without separating the “regular” and “irregular” parts of the daily magnetic field evolution. We demonstrate that the hourly gradient of the regular Sq variation is very small with respect to the irregular part, and a bulk of the nominal daily variation is actually part of the variation driven by solar wind and interplanetary magnetic field and traditionally classified as irregular. Therefore, attempts to subtract QDC can lead to a larger error, often caused by residual deviations between the used different mathematical and methodological tools and corresponding presumptions themselves. We show that ΔH provides the best and most consistent results at most timescales with the highest effective resolution among the studied indices. We also demonstrate that the ΔH index may equally be useful as a quick-look near-real-time index of space weather and as a long-term index derived from hourly magnetometer data for space climate studies.