W. Sun

Improvements to the HAF solar wind model for space weather predictions

We have assembled and tested, in real time, a space weather modeling system that starts at the Sun and extends to the Earth through a set of coupled, modular components. We describe recent efforts to improve the Hakamada-Akasofu-Fry (HAF) solar wind model that is presently used in our geomagnetic storm prediction system. We also present some results of these improvement efforts. In a related paper, Akasofu [2001] discusses the results of the first 2 decades using this system as a research tool and for space weather predictions. One key goal of our efforts is to provide quantitative forecasts of geoeffective solar wind conditions at the L1 satellite point and at Earth. Notably, we are addressing a key problem for space weather research: the prediction of the north-south component (Bz) of the interplanetary magnetic field. This parameter is important for the transfer of energy from the solar wind to the terrestrial environment that results in space weather impacts upon society. We describe internal improvements, the incorporation of timely and accurate boundary conditions based upon solar observations, and the prediction of solar wind speed, density, magnetic field, and dynamic pressure. HAF model predictions of shock arrival time at the L1 satellite location are compared with the prediction skill of the two operational shock propagation models: the interplanetary shock propagation model (ISPM) and the shock-time-of-arrival (STOA) model. We also show model simulations of shock propagation compared with interplanetary scintillation observations. Our modeling results provide a new appreciation of the importance of accurately characterizing event drivers and for the influences of the background heliospheric plasma on propagating interplanetary disturbances.

The average ionospheric electrodynamics for the different substorm phases

The average patterns of the electrostatic potential, current vectors, and Joule heating in the polar ionosphere, as well as the associated field-aligned currents, are determined for a quiet time, the growth phase, the expansion phase, the peak epoch, and the recovery phase of substorms. For this purpose, the Kamide-Richmond-Matsushita magnetogram-inversion algorithm is applied to a data set (for March 17, 18, and 19, 1978) from the six meridian magnetometer chains (the total number of magnetometer stations being 71) which were operated during the period of the International Magnetospheric Study (IMS). This is the first attempt at obtaining, on the basis of individual substorms, the average pattern of substorm quantities in the polar ionosphere for the different epochs. The main results are as follows : (1) The substorm-time current patterns over the entire polar region consist of two components. The first one is related to the two-cell convection pattern, and the second one is the westward electrojet in the dark sector which is related to the wedge current. (2) Time variations of the two components for the four substorm epochs are shown to be considerably different. (3) The dependence of these differences on the ionospheric electric field and the conductivities (Hall and Pedersen) is identified. (4) It is shown that the large-scale two-cell pattern in the electric potential is dominant during the growth phase of substorms. (5) The expansion phase is characterized by the appearance of a strong westward electrojet, which is added to the two-cell pattern. (6) The large-scale potential pattern becomes complicated during the recovery phase of substorms, but the two-cell pattern appears to be relatively dominant again during their late recovery as the wedge current subsides. These and many other earlier results are consistent with the present ones, which are more quantitatively and comprehensively demonstrated in our global study. Thus the two components are tentatively identified as the directly driven and the unloading components, respectively, although there is some indication that both components are actually coupled in the ionosphere. In the present paper we show that the directly driven component is present throughout the lifetime of substorms, becoming a dominant feature during the recovery phase of substorms as the unloading component wanes. The fact that the two components exist and that their time variations are different indicates that different processes are in progress even for the same value, say, −500 nT, of AL and that we must be cautious in using a single electrojet index, such as AL, in ordering satellite data.

Mathematical separation of directly driven and unloading components in the ionospheric equivalent currents during substorms

This paper attempts to separate objectively the directly driven and unloading components in substorm processes by applying the method of natural orthogonal components (MNOC). A time series of the ionospheric equivalent current function with time resolution of 5 min during March 17–19, 1978 is calculated on the basis of six meridian chains magnetometer data during the International Magnetospheric Study in order to obtain the fundamental orthogonal basis set. The first and second natural components of the set thus obtained dominate over the rest of the natural components. The first natural component is found to have a two-cell pattern, which is well known to be associated with global plasma convection in the magnetosphere. It is enhanced during the growth phase and expansion phase of substorms and decays during the recovery phase of substorms. Further, it is in fair correlation to the ϵ parameter with time lag of 20–25 min. This can be identified as the directly driven component. The second natural component reveals itself as an impulsive enhancement of the westward electrojet around midnight between 65° and 70° latitude during the expansion phase only. It is much less correlated with the ϵ parameter than the first one. Thus, as a first approximation, we identify it as the unloading component. It is shown that the directly driven component tends to dominate over the unloading component except for a brief period soon after substorm onset. This is the first clear determination of the time profile of the unloading component.

Substorm expansion phase caused by an intense localized convection imposed on the ionosphere

It is shown that the substorm expansion phase requires an intense localized convection in the midnight sector. The earlier magnetosphere-ionosphere coupling (MIC) model showed that the enhanced global convection imposed on the ionosphere can only produce the growth phase. By imposing the intense localized convection on the ionosphere at the end of the growth phase, the new MIC model presented in the present paper can quantitatively produce the signatures of all substorm phases. The pattern of the localized convection is consistent with the wedge-like field-aligned current obtained from the solution to the Alfven wave “field-current” equation. During the expansion phase in the new MIC model, the ionospheric convection field increases to ∼40 mV/m, the Hall conductance is enhanced to ∼33 mho, the upward field-aligned current increases to ∼1.8 μA/m2, and the auroral electrojet current increases to ∼0.8 × 106 A. The results of the MIC model are in good quantitative agreement with substorm observations. We suggest that the intense localized nightside convection is driven by localized reconnection of closed field lines in the plasma sheet. A remaining great challenge in substorm research is to uncover the cause of the localized plasma sheet reconnection.

A verification method for space weather forecasting models using solar data to predict arrivals of interplanetary shocks at Earth

The ability to predict the arrival of interplanetary shocks near earth is of great interest in space weather because of their relationship to sudden impulses and geomagnetic storms. A number of models have been developed for this purpose. For models to be used in forecasting, it is important to provide verification in the operational environment using standard statistical techniques because this enables the intercomparison of different models. A verification method is described here, comparing the prediction capabilities of four models that use solar observations for input. Three of the models are based on metric Type II radio burst observations, and one uses halo/partial-halo coronal mass ejections. A method of associating solar events with interplanetary shocks is described. The predictions are compared to associated shocks observed at L1 by the Advanced Composition Explorer (ACE) spacecraft. The time period of this study is January 2002-May 2002. Although the data sample is small, the statistical intercomparison of the results of these models is presented as a demonstration of the verification method.

Solar wind energy input during prolonged, intense northward interplanetary magnetic fields: A new coupling function

[1] Sudden energy release (ER) events in the midnight sector auroral zone during intense (B > 10 nT), long-duration (T > 3 h), northward (N = Bz > 0 nT) IMF magnetic clouds (MCs) during solar cycle 23 (SC23) have been examined in detail. The MCs with northward-then-southward (NS) IMFs were analyzed separately from MCs with southwardthen-northward (SN) configurations. It is found that there is a lack of ER/substorms during the N field intervals of NS clouds. In sharp contrast, ER events do occur during the N field portions of SN MCs. From the above two results it is reasonable to conclude that the latter ER events represent residual energy remaining from the preceding S portions of the SN MCs. We derive a new solar wind–magnetosphere coupling function during northward IMFs: ENIMF = a N −1/12 V 7/3 B 1/2 + b V |Dstmin|. The first term on the right-hand side of the equation represents the energy input via “viscous interaction,” and the second term indicates the residual energy stored in the magnetotail. It is empirically found that the magnetotail/magnetosphere/ionosphere can store energy for a maximum of ∼4 h before it has dissipated away. This concept is defining one for ER/substorm energy storage. Our scenario indicates that the rate of solar wind energy injection into the magnetotail/ magnetosphere/ionosphere for storage determines the potential form of energy release into the magnetosphere/ionosphere. This may be more important to understand solar wind–magnetosphere coupling than the dissipation mechanism itself (in understanding the form of the release). The concept of short-term energy storage is also applied for the solar case. It is argued that it may be necessary to identify the rate of energy input into solar magnetic loop systems to be able to predict the occurrence of solar flares.

An improved method to deduce the unloading component for magnetospheric substorms

Because our earlier analysis [Sun et al., 1998] confirmed that the directly driven component prevails during the growth phase, an attempt has been made in this paper to deduce the unloading component as accurately as possible. First, the directly driven pattern is determined by taking the average of patterns of the equivalent current function during the growth phase of substorms, then the correlation method is applied to calculate time variations of the magnitude of the directly driven component. Next, the method of natural orthogonal components is used to analyze the residual part, which equals the difference between the total current and the directly driven component. In the present method, the pattern and the magnitude of the unloading component are much improved. The correction in the magnitude of two components is ∼ 50%. Time variations of the magnitude of the unloading component thus obtained will be crucial in understanding the unloading process. The effect of the variable directly driven pattern, which is ∼ 11% of the magnitude of the directly driven component, also has been estimated quantitatively. This improved method has the advantage of allowing study of individual substorms over the earlier method.

Plane‐of‐sky simulations of interplanetary shock waves

[1] We present simulated plane-of-sky maps of the shock waves in interplanetary space from several representative solar events by using the Hakamada-Akasofu-Fry solar wind model (HAFv.2). This kinematic model uses a three-dimensional approach to construct plane-of-sky maps of interplanetary shock waves initiated by solar flares and CMEs. The simulated plane-of-sky maps are in a form that can be directly compared with assimilated optical observations of shock waves from Earth-orbiting and interplanetary satellites. Combining these simulations with observations would provide a new tool for monitoring the propagation of interplanetary shocks in interplanetary space and for predicting the arrival of shock waves at Earth.

Key Links to Space Weather: Forecasting Solar-Generated Shocks and Proton Acceleration

Forecasting the arrival of solar-generated shocks and accelerated protons anywhere in the heliosphere presents an awesome challenge in the new field of space weather. Currently, observations of solar wind plasmas and interplanetary magnetic fields are made at the sun-Earth libration point, L1, about 0.01 astronomical units (∼245 Earth radii) sunward of our planet. An obvious analogy is the pilot tube that protrudes ahead of a supersonic vehicle. The Advanced Composition Explorer and Solar and Heliospheric Observatory spacecraft, currently performing this function, provide about -1 h advance notice of impending arrival of interplanetary disturbances. The signatures of these disturbances may be manifested as interplanetary shock waves and/or coronal mass ejecta. We describe a first-generation procedure, based on first-principles numerical modeling, that provides the key links required to increase the advance notice (or lead time) to days, or even weeks. This procedure, instituted at the start of the present solar cycle 23, involves three separate models, used in real time, to predict the arrival of solar-event-initiated interplanetary shock waves at the L1 location. We present statistical results, using L1 observations as ground truth for 380 events. We also briefly discuss how one of these models (Hakamada-Akasofu-Fry version 2) may be used with a model that predicts the flux and fluence of energetic particles, for energies up to 100 MeV, that are generated by these propagating interplanetary shock waves.

Quantitative separation of the directly‐driven and unloading components of the ionospheric electric field

UL components for the substorms in March 17–19, 1978. However, equivalent currents are not real currents that can be detected. The separated DD and UL components are ambiguous in physics and cannot be confirmed by observations. [3] Statistical observations of the ionospheric electric field showed that in the auroral zone midnight sector the southward electric field (Es) suddenly enhanced up to � 30 mV/m from 0 and the westward electric field (Ew) jumped only � <5 mV/m at substorm expansion phase onset [Mozer, 1971]. The Es magnitude exceeded 30 mV/m, which was significantly higher than the increase of the Ew during substorm growth phase. Kamide and Kokubun [1996] also suggested that there are two major components in the ionospheric electric field, the Ew associated with magnetospheric convection and the Es associated with a substorm expansion phase. Kan [2007] proposed that an Es can be formed due to a blockage of the northward Hall currents produced by the Ew. The enhanced Es drives an intense westward Hall current in the ionosphere, i.e., the substorm westward electrojet. Its closure current in the plasma sheet may play an important role in causing a reduction and/or a disruption of the cross-tail current as well as a dipolarization of the magnetic field. Therefore, a quantitative description of Es and Ew is very important in understanding the development of substorms, as well as ring current intensification. However, an algorithm for such a purpose has not yet been developed. [4] In this letter we report a quantitative separation of the DD and UL components in the ionospheric electric field using the Natural Orthogonal Components (NOC) algorithm. As an example, we have studied a geomagnetic storm event on April 18, 2002, which is accompanied by a sawtooth event. The input data are AMIE electric potential in the high latitude ionosphere [Richmond et al., 1990]. Although there might be some uncertainty in the AMIE potential depending on the original dataset, our calculation results are very consistent with observations and previous studies.