A new reconstruction of the Dst index for 1932-2002 A. Karinen, K. Mursula To cite this version: A. Karinen, K. Mursula. A new reconstruction of the Dst index for 1932-2002. Annales Geophysicae, European Geosciences Union, 2005, 23 (2), pp.475-485. hal-00317550 HAL Id: hal-00317550 https://hal.archives-ouvertes.fr/hal-00317550 Submitted on 28 Feb 2005 HAL is a multi-disciplinary open access L’archive ouverte pluridisciplinaire HAL, est archive for the deposit and dissemination of sci- destinée au dépôt et à la diffusion de documents entific research documents, whether they are pub- scientifiques de niveau recherche, publiés ou non, lished or not. The documents may come from émanant des établissements d’enseignement et de teaching and research institutions in France or recherche français ou étrangers, des laboratoires abroad, or from public or private research centers. publics ou privés. Annales Geophysicae (2005) 23: 475–485 SRef-ID: 1432-0576/ag/2005-23-475 Annales © European Geosciences Union 2005 Geophysicae A new reconstruction of the Dst index for 1932–2002 A. Karinen and K. Mursula Department of Physical Sciences, P.O. Box 3000, FIN-90014 University of Oulu, Finland Received: 15 July 2004 – Revised: 7 October 2004 – Accepted: 18 October 2004 – Published: 28 February 2005 Abstract. We have reconstructed a new, homogeneous geo- 1 Introduction magnetic Dst index for 1932–2002, thus extending the orig- inal Dst index by 25 years, i.e. by more than one full so- The Dst index is traditionally calculated from the observa- lar magnetic cycle. The extension was done by using data tions at the four low-latitude magnetic field stations of Her- from the original set of four low-latitude stations for 1941– manus (HER), Honolulu (HON), Kakioka (KAK) and San 1956, and by using the nearby CTO station as a predecessor Juan (SJG). (For the coordinates and data coverages of the of the HER station for 1932–1940. Despite some open ques- magnetic stations used here, see Table 1). Although the mag- tions related to the composition of the original Dst index, the netic observations already started earlier at these stations the reconstructed index is quite similar to the original one dur- Dst index has been calculated only since the International ing the overlapping time interval (1957–2002). However, the Geophysical Year in 1957. reconstructed Dst index corrects for some known errors in At low latitudes the horizontal H component of magnetic the original Dst index, such as the erroneously large daily perturbation is mostly affected by the intensity of the equa- UT variation in 1971. Also, despite the overall agreement, torial ring current. Accordingly, the Dst index is calcu- the reconstructed index deviates from the original index even lated from the (normalized) values of this component. Ma- on the level of annual averages for several years. For in- jor disturbances in the Dst index during geomagnetic storms stance, all annual averages of the reconstructed index are are negative due to an increasing number of energetic parti- negative, and for 1962–1966 they are systematically lower cles carrying the ring current. Large amounts of energy are (more stormy) than those of the original index. Accordingly, fed into the inner magnetosphere, for example, in the form we disagree with the uniquely positive annual average of the of energetic particles, during long periods of southward di- original index in 1965, which most likely is erroneous. We rected interplanetary magnetic field (IMF). Once the IMF also find somewhat higher (less stormy) values than in the turns northward the ring current begins to decrease and the original Dst index for the three lowest annual averages in Dst index soon begins a slow rise back to its quiet time level. 1960, 1989 and 1991, out of which the lowest annual aver- Positive variations in the Dst index are mostly caused by age is found in 1989 rather than in 1991. The annual averages magnetospheric compressions due to interplanetary shocks of the geomagnetic Ap index and the reconstructed Dst in- often occurring in the initial phase of magnetic storms. In dex correlate very well over this time interval, except in the this phase, an abrupt increase in solar wind dynamic pres- beginning of the series in 1932–1940 and in the declining sure is often measured on the Earth’s surface as a sudden phase of solar cycles 18, 20 and 21, where high speed solar increase in magnetic intensity called the sudden storm com- wind streams cause enhanced geomagnetic activity. Using mencement (SSC). the superposed epoch method we also find that, on average, Note that rather than a homogeneous ring, the ring cur- the storms in the early extended period (1932–1956) are less rent is quite asymmetric and often consists of a number of intense but tend to have a longer recovery phase, suggest- longitudinally limited sections (Lui et al., 1987). Also, it ing that there are more HILDCAA-type medium activity in- has been known already for quite a long time that other cur- tervals during the early period than more recently. We also rent systems contribute to the magnetic variations even at low study the annually averaged storm structure over the 71-year latitudes and thereby also to the Dst index. This is simply time interval and find that the most stormy years occur dur- demonstrated by a recent result (Campbell, 2004) that the ing the declining phase of solar cycles 17 and 21 and around storm time disturbance at one longitude usually decreases the solar maxima of cycles 19 and 22. rather than increases with latitude. In particular, the day- side magnetopause current, which depends on solar wind Key words. Magnetospheric physics (Magnetospheric con- pressure, causes a contribution that is often subtracted from figuration and dynamics; Current systems; Storms and the D index when estimating the intensity of the ring cur- substorms) st rent (see, e.g. Burton et al., 1975; O’Brien and McPherron, 2000). Also, contributions to the Dst index by the tail cur- Correspondence to: A. Karinen rent and the field-aligned currents (Burton et al., 1975; Alex- (arto.karinen@oulu.fi) eev et al., 1996; Campbell, 1996; Turner et al., 2000) and 476 A. Karinen and K. Mursula: A new reconstruction of the Dst index for 1932–2002 Table 1. Geographical and geomagnetic coordinates, according to 2 Data quality and availability IGRF 2000 model (IGRF, 2000) and data coverages of the Dst mag- netic stations. As seen in Table 1 all four original Dst index stations (HER, HON, KAK, SJG) started operating already long before the Station, Geographic Geomagnetic Data international geophysical year (IGY) 1957 when the calcu- IAGA Code Lat. Long. Lat. Long. Availability lation of the original Dst index began (Sugiura and Kamei, Cape Town, CTO −34.57◦ 18.28◦ −33.89◦ 82.73◦ 1932–1940 1991; WDC-C2, 2004). Accordingly, we have reconstructed Hermanus, HER −34.42◦ 19.23◦ −33.91◦ 83.69◦ 1941–2002 Honolulu, HON 21.32◦ 202.00◦ 21.60◦ 269.45◦ 1902–2002 the new Dst index using data from these four stations since Kakioka, KAK 36.23◦ 140.18◦ 27.17◦ 208.50◦ 1913–2002 the start of the HER station in 1941 until the end of 2002. ◦ ◦ ◦ ◦ San Juan, SJG 18.12 293.15 28.53 5.87 1926–2002 Moreover, since HER was preceded by the nearby Cape Town (CTO) station we have calculated the new Dst index from 1932 onwards, using the observations at CTO for 1932– 1940 as a substitute for HER. (The exact starting time is currents induced in the ground (Hakkinen¨ et al., 2002) have 3 August 1932, at 00:00 UT). Taken into account the close been identified. Note also that the zero level of the Dst index proximity of HER and CTO the reconstructed Dst series will is not the physical zero level of the intensity of the ring cur- be quite homogeneous over the whole 71-year time interval. rent (Mayaud, 1978). Therefore, although the ring current is Also, the effects related to the imperfect hemispherical and among the dominant contributions to the Dst index, the exact longitudinal coverage of the Dst stations (like the UT vari- dependence between the Dst index and the various magneto- ation, see Takalo and Mursula, 2001a,b) will remain closely spheric current systems is quite complicated and will not be similar. Unfortunately, no intercalibration between HER and studied in this paper. However, this complicated interpreta- CTO could be made because no simultaneous observations tion does not decrease the value of the Dst index as an im- were available. portant measure of the global state of the magnetosphere and, in particular, of the development of magnetic storms. More- 2.1 Baseline steps over, the role of the Dst index as a long-term (solar cycle and inter-decennial) monitor of the magnetosphere is receiving As to data quality, we would like to note that there are some increasing interest. shifts in the baseline level of the H component at HON and In this paper we will reconstruct the Dst index following SJG. These shifts may be, for example due to an erroneous the original formula (see, e.g. Sugiura, 1964, 1969; Sugiura documentation of the baseline in the original annals of the and Kamei, 1991; WDC-C2, 2004) as closely as possible station. Figure 1 depicts the raw data of the HON station (see also Hakkinen¨ et al., 2003). Using the reconstructed which includes two steps in the baseline, the first on 1 April Dst index we find and correct some obvious errors and sus- 1947 at 21:00 UT and the latter on 1 May 1960 at 11:00 UT pectable features in the original Dst index.