OMNI Database Research Articles

Study of the solar Slow Sonic, Alfvén and Fast Magnetosonic transition surfaces

In this paper we study the shape, extend and time variations of the solar wind transition surfaces using the Lima and Priest (1993) hydrodynamic model adequately adapted for the case of the solar wind flow. The transition surfaces, namely the Slow (Sonic), the Alfvén, and the Fast Magnetosonic surface, are important boundaries around the Sun and play a crucial role in the development of the solar wind and the structure of the inner heliosphere. We determine the shape and dimension of these surfaces as a function of heliographic latitude using measurements from Ulysses spacecraft, and we also study their temporal variation using data from spacecrafts at 1 AU (OMNI database). Furthermore, we establish their dependence with the solar activity, demonstrating their shape and location for the last two solar cycles. From this we noticed that the temporal variation of all transition surfaces follows the 11-year solar cycle. Finally, from the OMNI database, we have studied the temporal variation over the past 40 years of the plasma β parameter, the kinetic to magnetic and the kinetic to thermal energy ratios, at a distance of 1 AU from the Sun.

Relative occurrence rate and geoeffectiveness of large-scale types of the solar wind

We investigate the relative occurrence rate for various types of the solar wind and their geoeffectiveness for magnetic storms with Dst < —50 nT. Both integrated effect for the entire time 1976–2000 and variations during this period of 2.5 cycles of solar activity are studied As raw data for the analysis we have used the catalog of large-scale types of the solar wind for the period 1976-2000 (see ftp://ftp.iki.rssi.ru/omni/) created by us with the use of the OMNI database (http://omni.web.gsgc.nasa.gov) [1] and described in detail in [2]. The average annual numbers of different type of events are as follows: 124 ±81 for the heliospheric current sheet (HCS), 8 ±6 for magnetic clouds (MC), 99 ±38 for Ejecta, 46 ±19 for Sheath before Ejecta, 6 ±5 for Sheath before MC, and 63 ±15 for CIR. The measurements that allowed one to determine a source in the solar wind were available only for 58% of moderate and strong magnetic storms (with index Dst < —50 nT) during the period 1976–2000. Magnetic clouds (MC) are shown to be the most geoeffective (~61%). The CIR events and Ejecta with Sheath region are three times less geoeffective (~20–21 %). Variations of occurrence rate and geoeffectiveness of various types of the solar wind in the solar cycle are discussed.

Reliability of prediction of the magnetosheath BZ component from interplanetary magnetic field observations

In the present statistical study, we discuss a probability of simultaneous observations of the same sign of the magnetic field BZ component in the solar wind and magnetosheath. The analysis is based on 5 min data from four spacecraft (Interball‐1, IMP 8, Cluster, and THEMIS) operating in different phases of the solar cycle in the magnetosheath. Their measurements are compared with Wind interplanetary magnetic field (IMF) observations, and other available upstream monitors (ACE, THEMIS B, and OMNI database) are tested for some sets. We can conclude that the probability of observations of the same BZ sign in the solar wind and in the magnetosheath is surprisingly very low from a general point of view. The probability changes through the solar cycle, being larger at the solar minimum. Regardless of the solar cycle phase, this probability is close to 0.5 (random coincidence) for IMF ∣BZ∣ &lt; 1 nT, and it is a rising function of the BZ value. Distant solar wind monitors do not guarantee the same sign of the BZ component, even for values of IMF BZ exceeding ±9 nT, but such large values are observed about 3–5% of the time. A better probability profile is reached for a monitor located just upstream (less than 30 RE), as is demonstrated for the THEMIS project.

Reverse convection potential saturation during northward IMF under various driving conditions

We report the results of an investigation of the reverse convection potentials in the dayside high‐latitude ionosphere during periods of steady northward interplanetary magnetic field (IMF). While it has been demonstrated that the reverse convection potentials exhibit saturation behavior, an investigation into how the saturated reverse convection cells respond to various driving conditions has not yet been performed. We use the OMNI database from 1998 to 2007 to search for events in solar wind data propagated to the bow shock when the IMF is northward and when various driving parameters are stable for more than 40 min. We then use bin‐averaged SuperDARN Doppler radar velocity data to apply a spherical harmonic fit to calculate the average potential pattern for various driving conditions. Results show that the saturated reverse convection potential during northward IMF is substantially lower than during southward IMF (as expected), but the strength of the saturation electric field at a particular location is comparable during northward IMF to what it is during southward IMF. Seasonal dependence and dependence on the Alfvènic Mach number and solar wind β are also investigated. It appears that the reverse convection potential demonstrates the opposite of what is expected for seasonal dependence, and that the Alfvènic Mach number is not the fundamental driver of its saturation.

Solar Wind Sources in the Late Declining Phase of Cycle 23: Effects of the Weak Solar Polar Field on High Speed Streams

The declining phases of solar cycles are known for their high speed solar wind streams that dominate the geomagnetic responses during this period. Outstanding questions about these streams, which can provide the fastest winds of the solar cycle, concern their solar origins, persistence, and predictability. The declining phase of cycle 23 has lasted significantly longer than the corresponding phases of the previous two cycles. Solar magnetograph observations suggest that the solar polar magnetic field is also ∼ 2 – 3 times weaker. The launch of STEREO in late 2006 provided additional incentive to examine the origins of what is observed at 1 AU in the recent cycle, with the OMNI data base at the NSSDC available as an Earth/L1 baseline for comparisons. Here we focus on the year 2007 when the solar corona exhibited large, long-lived mid-to-low latitude coronal holes and polar hole extensions observed by both SOHO and STEREO imagers. STEREO provides in situ measurements consistent with rigidly corotating solar wind stream structure at up to ∼ 45° heliolongitude separation by late 2007. This stability justifies the use of magnetogram-based steady 3D solar wind models to map the observed high speed winds back to their coronal sources. We apply the WSA solar wind model currently running at the NOAA Space Weather Prediction Center with the expectation that it should perform its best at this quiet time. The model comparisons confirm the origins of the observed high speed streams expected from the solar images, but also reveal uncertainties in the solar wind source mapping associated with this cycle’s weaker solar polar fields. Overall, the results illustrate the importance of having accurate polar fields in synoptic maps used in solar wind forecast models. At the most fundamental level, they demonstrate the control of the solar polar fields over the high speed wind sources, and thus one specific connection between the solar dynamo and the solar wind character.

Catalog of large-scale solar wind phenomena during 1976–2000

The main goal of this paper is to compile a catalog of large-scale phenomena in the solar wind over the observation period of 1976-2000 using the measurement data presented in the OMNI database. This work included several stages. At first the original OMNI database was supplemented by certain key parameters of the solar wind that determine the type of the solar wind stream. The following parameters belong to this group: the plasma ratio β , thermal ( NkT ) and kinetic ( mNV 2 ) pressures of the solar wind, the ratio T / T exp of measured and expected temperatures, gradients of the plasma velocity and density, and the magnetic field gradient. The results of visualization of basic plasma parameters that determine the character of the solar wind stream are presented on the website of the Space Research Institute, Moscow. Preliminary identification of basic types of the solar wind stream (FAST and SLOW streams, Heliospheric Current Sheet (HCS), Corotating Interaction Region (CIR), EJECTA (or Interplanetary Coronal Mass Ejections), Magnetic Cloud (MC), SHEATH (compression region before EJECTA/MC), rarified region RARE, and interplanetary shock wave IS) had been made with the help of a preliminary identification program using the preset threshold criteria for plasma and interplanetary magnetic field parameters. Final identification was done by comparison with the results of visual analysis of the solar wind data. In conclusion, histograms of distributions and statistical characteristics are presented for some parameters of various large-scale types of the solar wind.

Statistical investigation of heliospheric conditions resulting in magnetic storms

Time behavior of the solar wind and interplanetary magnetic field parameters is investigated for 623 magnetic storms of the OMNI database for the period 1976–2000. The analysis is carried out by the superposed epoch technique (the magnetic storm onset time is taken to be the beginning of an epoch) for five various categories of storms induced by various types of solar wind: CIR (121 storms), Sheath (22 storms), MC (113 storms), and “uncertain type” (367 storms). In total, the analysis conducted for “all storms” included 623 storms. The obtained data, on one hand, confirm the results obtained earlier without selecting the intervals according to the solar wind types, and, on the other hand, they indicate the existence of distinctions in the time variation of parameters for various types of solar wind. Though the lowest values of the B z-component of IMF are observed in the MC, the lowest values of the D st-index are achieved in the Sheath. Thus, the strongest magnetic storms are induced, on average, during the Sheath rather than during the MC body passage, probably owing to higher pressure in the Sheath. Higher values of nkT, T/T exp, and β parameters are observed in the CIR and Sheath and lower ones in the MC, which corresponds to the physical essence of these solar wind types.

Influence of the solar wind dynamic pressure on the decay and injection of the ring current

The influence of the solar wind dynamic pressure on the decay and injection of the ring current is investigated empirically, on the basis of the solar wind and the geomagnetic index Dst of the OMNI database, for the period from January 1964 to July 2001. We found that when the position of the ring current is closer to the Earth for a higher solar wind dynamic pressure, the decay time of the ring current decreases. The decay time, in hours, varies as follows, τ = 8.70 exp(6.66/(6.04 + P)), for northward interplanetary magnetic fields (IMF), where P is the solar wind dynamic pressure in nanopascals. It is also found, by minimizing the root mean square errors of the hourly Dst difference between the calculated values and the measured ones, that the ring current injection rate is proportional to the solar wind dynamic pressure, with a power index equal to 0.2 during southward IMF. This implies that the ring current injection increases when the magnetosphere is more compressed by high solar wind dynamic pressure. On the basis of our new results we demonstrate that the predictions of Dst using O'Brien and McPherron's [2000a] model are improved, especially for intense geomagnetic storms when the influence of the solar wind dynamic pressure on the decay and injection of ring current is taken into consideration.

Wind and ACE observations during the great flow of 1–4 May 1998: Relation to solar activity and implications for the magnetosphere

Using magnetic field, plasma, and energetic particle data from Wind and ACE, we analyze interplanetary features associated with the first strongly geoeffective interval in the rising phase of solar cycle 23, which affected Earth on 1–4 May 1998. As shown by Skoug et al. [1999], the configuration consisted of a compound stream made up of an interplanetary coronal mass ejection (ICME) containing a magnetic cloud and being trailed by a hot, faster flow. In addition, we find that the front boundary of the ICME is a rotational discontinuity and the leading edge of the fast stream has a zero normal magnetic field component and is followed by a magnetic field which is strongly enhanced (by a factor of ∼4) and whose fluctuations lie in a plane approximately parallel to the leading edge. Energetic particle and composition observations confirm that the field lines of the magnetic cloud were connected to at least two different flare sites in the same active region. We infer a lower limit for the size of the solar footprint of the connected flux tube of 0.02 Rs2, i.e., ∼1010 km2. A dramatic weakening of the halo electron distribution occurred during 3 May at the time when other experimenters have documented the presence of prominence material. We hypothesize that the solar wind halo population was scattered by enhanced frequency of Coulomb collisions in the dense and very cold plasma. We discuss our energetic particle observations in terms of local acceleration at interplanetary shocks and field discontinuities as well as in terms of acceleration in flares and CME‐driven shocks. We also compare, in a specific formulation, the power and energy input of the May 1998 configuration to the magnetosphere with other much studied geoeffective events. We find that the power input far exceeded that in all previous geoeffective events in our sample and attribute this to the fact that the 1–4 May 1998 event consisted of a compound stream structure with an unprecedented power input during a ∼3‐hour burst of high‐speed flow on 4 May. A statistical survey using the OMNI database for the 6‐year period 1995–2000 confirms these inferences and indicates further approximate saturation levels for energy and power input of 10 J m−2 and 0.3 mW m−2, only exceeded in exceptional events such as May 1998 and July 2000. The solar energetic particle event at the leading edge of the fast stream might be the only advance warning the Earth would receive of the approach of a configuration of such a concentrated geoeffective potential.

Solar‐cycle variation of low density solar wind during more than three solar cycles

The May, 1999 low density (&lt;1 cm−3) solar wind interval is one of a series of intervals of low density solar wind which have been detected since in‐situ, near‐Earth observations began. Examining the NSSDC OMNI database since 1965, covering solar Cycles 20–23, we show that such intervals, which are also periods of unusually low mass flux and low dynamic pressure, occur most frequently around sunspot maximum and are rarer at solar minimum. The occurrence rate of low‐density plasma may be higher in weaker sunspot cycles (Cycle 20 and the current Cycle 23). Around two‐thirds of periods with densities ≤1 cm−3 are associated with transient solar wind structures, in particular with ejecta and post‐shock flows. The majority of other events are associated with corotating streams. The May 1999 event is unusual because it is not associated with an ejecta or stream. A similar period was observed in July–August 1979.

An empirical phase space analysis of ring current dynamics: Solar wind control of injection and decay

This empirical analysis of the terrestrial ring current, as measured by Dst, uses conditional probability density in Dst phase space to determine the evolution of the ring current. This analysis method does not assume a dynamic equation, but merely requires that the evolution of Dst depends on Dst and the solar wind. Our simple model, with seven nontrivial parameters, describes the dynamics of 30 years of hourly Dst with solar wind data provided by the OMNI database. The solar wind coupling is assumed to be determined by VBs. We arrive at a dynamic equation nearly identical to the Burton equation (Burton et al., 1975) with a slight correction. The method is restricted to Dst &gt; −150 nT owing to the rarity of larger excursions. We show that the ring current decay lifetime varies with VBs but not with Dst, and we relate this variation to the position of convection boundaries in the magnetosphere. Convection boundaries closer to the Earth result in shorter charge exchange decay times owing to the higher neutral density near the Earth. The decay time in hours varies as τ = 2.40 exp [9.74/(4.69+VBs)] with VBs in millivolts per meter. We also show that the energy injection function as derived by Burton et al. is essentially correct. The injection Q is zero for VBs &lt; Ec = 0.49 mV/m, and it is Q = −4.4(VBs − Ec) for VBs &gt; Ec. We derive the correction for magnetopause contamination: Dst* = Dst −7.26P1/2 + 11 nT, where P is solar wind dynamic pressure in nanopascals. Finally, we apply the model to a moderate storm and to an intense storm. We demonstrate that, in spite of the fact that spacecraft observe compositional changes in the ring current at intense Dst, the dynamics of the two storms are not obviously different in the context of our model. We demonstrate that the generally observed dependence of the decay parameter on Dst is actually an alias of the coincidence of intense Dst and intense VBs.

Prediction of magnetic storms by nonlinear models

The strong correlation between magnetic storms and southward interplanetary magnetic field (IMF) is well known from linear prediction filter studies using the Dst and IMF data. However, the linear filters change significantly from one storm to another and thus are limited in their predicting ability. Previous studies have indicated nonlinearity in the magnetospheric response as the ring current decay rate varies with the Dst value during storms. We present in this letter nonlinear models for the evolution of the Dst based on the OMNI database for 1964–1990. When solar wind data are available in advance, the evolution of storms can be predicted from the Dst and IMF data. Solar wind data, however, are not available most of the time or are available typically an hour or less in advance. Therefore, we have developed nonlinear predictive models based on the Dst data alone. In the absence of solar wind data, these models cannot predict the storm onset, but can predict the storm evolution, and may identify intense storms from moderate ones. The input‐output model based on IMF and Dst data, the autonomous model based on Dst alone, and a combination of the two can be used as forecasting tools for space weather.