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Crooker, N. U.

Publications and source records attributed to Crooker, N. U..

At least 19 records
All sourcesNASA NTRSDOE OSTI
NASA NTRS2012

Global Network of Slow Solar Wind

The streamer belt region surrounding the heliospheric current sheet (HCS) is generally treated as the primary or sole source of the slow solar wind. Synoptic maps of solar wind speed predicted by the Wang-Sheeley-Arge model during selected periods of solar cycle 23, however, show many areas of slow wind displaced from the streamer belt. These areas commonly have the form of an arc that is connected to the streamer belt at both ends. The arcs mark the boundaries between fields emanating from different coronal holes of the same polarity and thus trace the paths of belts of pseudostreamers, i.e., unipolar streamers that form over double arcades and lack current sheets. The arc pattern is consistent with the predicted topological mapping of the narrow open corridor or singular separator line that must connect the holes and, thus, consistent with the separatrix-web model of the slow solar wind. Near solar maximum, pseudostreamer belts stray far from the HCS-associated streamer belt and, together with it, form a global-wide web of slow wind. Recognition of pseudostreamer belts as prominent sources of slow wind provides a new template for understanding solar wind stream structure, especially near solar maximum.

solar wind↗
NASA NTRS2010

Role of the Russell-McPherron Effect in the Acceleration of Relativistic Electrons

While it is well known that high fluxes of relativistic electrons in the Earth's radiation belts are associated with high-speed solar wind and its heightened geoeffectiveness,less known is the fact that the Russell McPherron(R M) effect strongly controls whether or not a given high-speed stream is geoffective. To test whether it then follows that the R M effect also strongly controls fluxes of relativistic electrons, we perform a superposed epoch analysis across corotating interaction regions (CIR) keyed on the interfaces between slow and fast wind. A total of 394 stream interfaces were identified in the years 1994-2006. Equinoctial interfaces were separated into four classes based on the R-M effect,that is, whether the solar wind on either side of the interface was either(geo)effective (E) or ineffective (I) depending on season and the polarity of the interplanetary magnetic field (IMF). Four classes of interface identified as II, IE, EI,and EE are possible. The classes IE and EI correspond to CIRs with polarity changes indicating passage through the heliospheric current sheet. To characterize the behavior of solar wind and magnetospheric variables, we produced maps of dynamic cumulative probability distribution functions (cdfs) as a function of time over 10-day intervals centered on the interfaces. These reveal that effective high-speed streams have geomagnetic activity nearly twice as strong as ineffective streams and electron fluxes a factor of 12 higher. In addition they show that an effective low-speed stream increases the flux of relativistic electrons before the interface so that an effective to ineffective transition results in lower fluxes after the interface.We conclude that the R-M effect plays a major role in organizing and sustaining a sequence of physical processes responsible for the acceleration of relativistic electrons.

McPherron, R. L.↗
NASA NTRS2008

Evidence in Magnetic Clouds for Systematic Open Flux Transport on the Sun

Most magnetic clouds encountered by spacecraft at 1 AU display a mix of unidirectional suprathermal electrons signaling open field lines and counterstreaming electrons signaling loops connected to the Sun at both ends. Assuming the open fields were originally loops that underwent interchange reconnection with open fields at the Sun, we determine the sense of connectedness of the open fields found in 72 of 97 magnetic clouds identified by the Wind spacecraft in order to obtain information on the location and sense of the reconnection and resulting flux transport at the Sun. The true polarity of the open fields in each magnetic cloud was determined from the direction of the suprathermal electron flow relative to the magnetic field direction. Results indicate that the polarity of all open fields within a given magnetic cloud is the same 89% of the time, implying that interchange reconnection at the Sun most often occurs in only one leg of a flux rope loop, thus transporting open flux in a single direction, from a coronal hole near that leg to the foot point of the opposite leg. This pattern is consistent with the view that interchange reconnection in coronal mass ejections systematically transports an amount of open flux sufficient to reverse the polarity of the heliospheric field through the course of the solar cycle. Using the same electron data, we also find that the fields encountered in magnetic clouds are only a third as likely to be locally inverted as not. While one might expect inversions to be equally as common as not in flux rope coils, consideration of the geometry of spacecraft trajectories relative to the modeled magnetic cloud axes leads us to conclude that the result is reasonable.

Crooker, N. U.↗
NASA NTRS2008

Estimating Total Heliospheric Magnetic Flux from Single-Point in Situ Measurements

A fraction of the total photospheric magnetic flux opens to the heliosphere to form the interplanetary magnetic field carried by the solar wind. While this open flux is critical to our understanding of the generation and evolution of the solar magnetic field, direct measurements are generally limited to single-point measurements taken in situ by heliospheric spacecraft. An observed latitude invariance in the radial component of the magnetic field suggests that extrapolation from such single-point measurements to total heliospheric magnetic flux is possible. In this study we test this assumption using estimates of total heliospheric flux from well-separated heliospheric spacecraft and conclude that single-point measurements are indeed adequate proxies for the total heliospheric magnetic flux, though care must be taken when comparing flux estimates from data collected at different heliocentric distances.

Owens, M. J.↗
NASA NTRS2008

The Heliospheric Magnetic Field Over the Hale Cycle

The concept that open magnetic flux of the Sun (rooted with one and only one footpoint at the Sun) is a conserved quantity is taking root in the heliospheric community. Observations show that the Sun's open magnetic flux returns to the baseline from one solar minimum to the next. The temporary enhancement in the 1AU heliospheric magnetic flux near solar maximum can be accounted for by the temporary creation of closed magnetic flux (with two footpoints at the Sun) during the ejection of coronal mass ejections (CMEs), which are more frequent near solar maximum. As a part of the International Heliophysical Year activities, this paper reviews two recently discussed consequences of open flux conservation: the reversal of open magnetic flux over the solar cycle driven by Coronal Mass Ejections and the impacts of open flux conservation on the global structure of the heliospheric magnetic field. These studies demonstrate the inherent linkages between coronal mass ejections, footpoint motions back at the Sun, and the global structure and evolution of the heliospheric magnetic field.

Schwardron, N. A.↗
NASA NTRS2008

Residual Strahls in Solar Wind Electron Dropouts: Signatures of Magnetic Connection to the Sun, Disconnection, or Interchange Reconnection?

A recent assessment of suprathermal electron heat flux dropouts (HFDs) in the solar wind eliminated 90% as possible signatures of field lines disconnected from the Sun at both ends (Pagel et al., 2005b). The primary reason for elimination was the presence of a residual field-aligned strahl presumably signaling field lines connected to the Sun. Using high-time-resolution data from the Wind spacecraft, this paper tests whether the residual strahls were an artifact of averaging over pitch angle distributions (PADs) with and without strahls. An automated search for PADs without strahls (flat PADs) yields an occurrence rate of only 14% within HFDs, but a detailed case study shows that these flat PADs are imbedded within intervals of nearly flat PADs, that is, PADS with residual strahls that cannot be artifacts of averaging. An attractive alternative is that the residual strahls result from intermixing of originally back-scattered fluxes (haloes) of unequal intensities on field lines that have either disconnected or interchange reconnected at the Sun. A reevaluation of reported streaming of higher-energy electrons in HFDs suggests a similar cause. While the high-time-resolution data show high variability of PAD profiles within HFDs, this paper reopens the possibility that a substantial fraction signal disconnection or interchange reconnection. Estimated occurrence rates of fields having undergone these processes based upon published HFD rates are of the same order of magnitude as the surprisingly low values of 1-5% recently predicted by a model of a balanced heliospheric flux budget (Owens and Crooker, 2007).

Crooker, N. U.↗
NASA NTRS2007

Role of Coronal Mass Ejections in the Heliospheric Hale Cycle

The 11-year solar cycle variation in the heliospheric magnetic field strength can be explained by the temporary buildup of closed flux released by coronal mass ejections (CMEs). If this explanation is correct, and the total open magnetic flux is conserved, then the interplanetary-CME closed flux must eventually open via reconnection with open flux close to the Sun. In this case each CME will move the reconnected open flux by at least the CME footpoint separation distance. Since the polarity of CME footpoints tends to follow a pattern similar to the Hale cycle of sunspot polarity, repeated CME eruption and subsequent reconnection will naturally result in latitudinal transport of open solar flux. We demonstrate how this process can reverse the coronal and heliospheric fields, and we calculate that the amount of flux involved is sufficient to accomplish the reversal within the 11 years of the solar cycle.

Owens, M. J.↗
NASA NTRS2004

KINEMATIC TREATMENT OF CORONAL MASS EJECTION EVOLUTION IN THE SOLAR WIND

We present a kinematic study of the evolution of coronal mass ejections (CMEs) in the solar wind. Specifically, we consider the effects of (1) spherical expansion and (2) uniform expansion due to pressure gradients between the interplanetary CME (ICME) and the ambient solar wind. We compare these results with an MHD model that allows us to isolate these effects h m the combined kinematic and dynamical effects, which are included in MHD models. They also provide compelling evidence that the fundamental cross section of so-called "force-free" flux ropes (or magnetic clouds) is neither circular or elliptical, but rather a convex-outward, "pancake" shape. We apply a force-free fit to the magnetic vectors from the MHD simulation to assess how the distortion of the flux rope affects the fit. In spite of these limitations, force-free fits, which are straightforward to apply, do provide an important description of a number of parameters, including the radial dimension, orientation, and chirality of the ICME. Subject headings: MHD - solar wind - Sun: activity - Sun: corona - Sun: coronal mass ejections (CMEs) - On-line material color figures Sun: magnetic fields

Riley, Pete↗
NASA NTRS2004

Ulysses Observations of the Magnetic Connectivity between CMEs and the Sun

We have investigated the magnetic connectivity of coronal mass ejections (CMEs) to the Sun using Ulysses observations of suprathermal electrons at various distances between 1 AU and 5.2 AU. Drawing on ideas concerning the eruption and evolution of CMEs, we had anticipated that there might be a tendency for CMEs to contain progressively more open field lines, as reconnection back at the Sun either opened or completely disconnected previously closed field lines threading the CMEs. Our results, however, did not yield any discernible trend. By combining the potential contribution of CMEs to the heliospheric flux with the observed build-up of flux during the course of the solar cycle we also derive a lower limit for the reconnection rate of CMEs that is sufficient to avoid the "flux catastrophe" paradox. This rate is well below our threshold of detectability.

Riley, Pete↗
NASA NTRS2004

Kinematic Treatment of CME Evolution in the Solar Wind

We present a kinematic study of the evolution of coronal mass ejections (CMEs) in the solar wind. Specifically, we consider the effects of: (1) spherical expansion; and (2) uniform expansion due to pressure gradients between the Interplanetary CME (ICME) and the ambient solar wind. We compare these results with an MHD model, which allows us to isolate these effects from the combined kinematic and dynamical effects, which are included in MHD models. They also provide compelling evidence that the fundamental cross section of so-called "force-free" flux ropes (or magnetic clouds) is neither circular or elliptical, but rather a convex-outward, "pancake" shape. We apply a force-free fitting to the magnetic vectors from the MHD simulation to assess how the distortion of the flux rope affects the fitting. In spite of these limitations, force-free fittings, which are straightforward to apply, do provide an important description of a number of parameters, including the radial dimension, orientation and chirality of the ICME.

Riley, Pete↗
NASA NTRS1995

The heliospheric sector boundary as a distented magnetic cloud

A magnetic cloud was detected both near Earth and by Pioneer 11 located 43 deg east of Earth at 4.8 AU. The magnetic field within the cloud rotated smoothly from toward to away polarity, marking sector boundary passage. Interpreted as a flux rope, the cloud had a vertical axis, implying that its cylindrical cross-section in the ecliptic plane was distended along the sector boundary by at least 43, forming an extensive occlusion in the heliospheric current sheet. At 1 AU the cloud had plasma signatures typical of a fast coronal mass ejection with low temperature and a leading shock. In contrast, at 4.8 AU, only the cloud signature remained. Its radial dimension was the same at both locations, consistent with little expansion beyond 1 AU. Energetic particle data at 4.8 AU show high fluxes preceding the cloud but not extending forward to the corotating shock that marked entry into the interaction region containing the cloud. The streaming direction was antisunward, consistent with possible acceleration in a low-beta region of field line draping around the cloud's western (upstream) end. The fluxes dropped upon entry into the cloud and became essentially isotropic one third of the way through it. On the basis of sector boundary characteristics published in the past, we suggest that distended clouds may be common heliospheric current sheet occlusions.

Crooker, N. U.↗
NASA NTRS1994

Heliospheric current sheet inclinations predicted from source surface maps

The inclinations of the neutral line at the ecliptic plane derived from source surface model maps of coronal fields are measured for the interval from June 1976 to March 1992. The mean and median values of 53 deg and 57 deg are close to the average inclinations determined earlier from minimum variance analyses of solar wind measurements at sector boundaries, but the mode falls in the 80 deg - 90 deg bin. This result, which is based on the model assumptions implicit in deriving the source surface maps, predicts that the heliospheric current sheet typically intersects the ecliptic plane nearly at right angles, even without steepening by stream interaction regions. High inclinations dominate the solar cycle for about 7 years around solar maximum. Dips to lower inclination occur near solar minimum, but high variance admits a wide range of inclinations throughout the cycle. Compared to the smooth solar cycle variation of the maximum latitudinal excursion of the neutral line, often treated as the tilt angle of a flat heliospheric current sheet, the noisy variation of the inclinations reflects the degree to which the neutral line deviates from a sine wave, implying warps and corrugations in the current sheet. About a third of the time the neutral line so deviates that it doubles back in longitude.

Shodhan, S.↗
NASA NTRS1994

A test of source-surface model predictions of heliospheric current sheet inclination

The orientation of the heliospheric current sheet predicted from a source surface model is compared with the orientation determined from minimum-variance analysis of International Sun-Earth Explorer (ISEE) 3 magnetic field data at 1 AU near solar maximum. Of the 37 cases analyzed, 28 have minimum variance normals that lie orthogonal to the predicted Parker spiral direction. For these cases, the correlation coefficient between the predicted and measured inclinations is 0.6. However, for the subset of 14 cases for which transient signatures (either interplanetary shocks or bidirectional electrons) are absent, the agreement in inclinations improves dramatically, with a correlation coefficient of 0.96. These results validate not only the use of the source surface model as a predictor but also the previously questioned usefulness of minimum variance analysis across complex sector boundaries. In addition, the results imply that interplanetary dynamics have little effect on current sheet inclination at 1 AU. The dependence of the correlation on transient occurrence suggests that the leading edge of a coronal mass ejection (CME), where transient signatures are detected, disrupts the heliospheric current sheet but that the sheet re-forms between the trailing legs of the CME. In this way the global structure of the heliosphere, reflected both in the source surface maps and in the interplanetary sector structure, can be maintained even when the CME occurrence rate is high.

Burton, M. E.↗
NASA NTRS1993

Ionospheric convection response to slow, strong variations in a Northward interplanetary magnetic field: A case study for January 14, 1988

We analyze ionospheric convection patterns over the polar regions during the passage of an interplanetary magnetic cloud on January 14, 1988, when the interplanetary magnetic field (IMF) rotated slowly in direction and had a large amplitude. Using the assimilative mapping of ionospheric electrodynamics (AMIE) procedure, we combine simultaneous observations of ionspheric drifts and magnetic perturbations from many different instruments into consistent patterns of high-latitude electrodynamics, focusing on the period of northward IMF. By combining satellite data with ground-based observations, we have generated one of the most comprehensive data sets yet assembled and used it to produce convection maps for both hemispheres. We present evidence that a lobe convection cell was embedded within normal merging convection during a period when the IMF B(sub y) and B(sub z) components were large and positive. As the IMF became predominantly northward, a strong reversed convection pattern (afternoon-to-morning potential drop of around 100 kV) appeared in the southern (summer) polar cap, while convection in the northern (winter) hemisphere became weak and disordered with a dawn-to-dust potential drop of the order of 30 kV. These patterns persisted for about 3 hours, until the IMF rotated significantly toward the west. We interpret this behavior in terms of a recently proposed merging model for northward IMF under solstice conditions, for which lobe field lines from the hemisphere tilted toward the Sun (summer hemisphere) drape over the dayside magnetosphere, producing reverse convection in the summer hemisphere and impeding direct contact between the solar wind and field lines connected to the winter polar cap. The positive IMF B(sub x) component present at this time could have contributed to the observed hemispheric asymmetry. Reverse convection in the summer hemisphere broke down rapidly after the ratio absolute value of B(sub y)/B(sub z) exceeded unity, while convection in the winter hemisphere strengthened. A dominant dawn-to-dusk potential drop was established in both hemispheres when the magnitude of B(sub y) exceeded that of B(sub z) with potential drops of the order of 100 kV, even while B(sub z) remained northward. The latter transition to southward B(sub z) produced a gradual intensification of the convection, but a greater qualitative change occurred at the transition through absolute value of B(sub y)/B(sub z) = 1 than at at the transition through B(sub z) = 0. The various convection patterns we derive under northward IMF conditions illustrate all possibilities previously discussed in the literature: nearly single-cell and multicell, distorted and symmetric, ordered and unordered, and sunward and antisunward.

Knipp, D. J.↗
NASA NTRS1993

Multiple heliospheric current sheets and coronal streamer belt dynamics

The occurrence of multiple directional discontinuities in the coronal streamer belt at sector boundary crossings in the heliosphere, often ascribed to waves or kinks in the heliospheric current sheet, may alternatively be attributed to a network of extended current sheets from multiple helmet streamers with a hierarchy of sizes at the base of the corona. Frequent transient outflows from these helmets can account for a variety of signatures observed at sector boundaries, including ordered field rotations, planar magnetic structures and sandwichlike plasma structure.

Crooker, N. U.↗
NASA NTRS1992

The semiannual variation of great geomagnetic storms and the postshock Russell-Mcpherron effect preceding coronal mass ejecta

Recent results indicate that the intense southward interplanetary magnetic fields (IMFs) responsible for great storms can reside in the postshock plasma preceding the driver gas of coronal mass ejections (CMEs) as well as in the driver gas itself. It is proposed here that strong southward fields in the postshock flow result from a major increase in the Russell-McPherron polarity effect through a systematic pattern of compression and draping within the ecliptic plane. Differential compression at the shock increases the Parker spiral angle and, consequently, the azimuthal field component that projects as a southward component onto earth's dipole axis. The resulting prediction is that southward fields in the postshock plasma maximize at the spring (fall) equinox in CMEs emerging from toward (away) sectors. This pattern produces a strong semiannual variation in postshock IMF orientation and may account at least in part for the observed semiannual variation of the occurrence of great geomagnetic storms.

Crooker, N. U.↗