Ulysses observations of very different heliospheric structure during the declining phase of solar activity cycle 23

McComas, D. J.; Elliott, H. A.; Gosling, J. T.; Skoug, R. M.

doi:10.1029/2006gl025915

Cited by 49 publications

(48 citation statements)

References 14 publications

(16 reference statements)

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“…The solar polar field strength measured by the Wilcox Solar Observatory for cycle 23 was reported as being ∼50% lower than for the previous three solar cycles. McComas et al (2008) have indicated that the solar wind emanating from the large polar coronal holes is slightly slower (∼3%), significantly less dense (∼17%), and cooler (∼14%), with less mass and momentum flux (∼20%) than previous solar cycle minimum values, which has a broad effect on the structure and size of the heliosphere. Infrared spectral observations of sunspots from 1998 to 2011 have shown that the strength of the magnetic fields of the sunspots has gradually weakened and the temperatures have risen (Livingston et al 2012).…”

Section: Introductionmentioning

confidence: 90%

Periodicities in the X-Ray Emission From the Solar Corona

Chowdhury

Jain

Awasthi

2013

ApJ

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We have studied the time series of full disk integrated soft and hard X-ray emission from the solar corona during 2004 January to 2008 December, covering the entire descending phase of solar cycle 23 from a global point of view. We employ the daily X-ray index derived from 1 s cadence X-ray observations from the Si and CZT detectors of the "Solar X-ray Spectrometer" mission in seven different energy bands ranging between 6 and 56 keV. X-ray data in the energy bands 6-7, 7-10, 10-20, and 4-25 keV from the Si detector are considered, while 10-20, 20-30, and 30-56 keV high energy observations are taken from the CZT detector. The daily time series is subjected to power spectrum analysis after appropriate correction for noise. The Lomb-Scargle periodogram technique has shown prominent periods of ∼13.5 days, ∼27 days, and a near-Rieger period of ∼181 days and ∼1.24 yr in all energy bands. In addition to this, other periods like ∼31, ∼48, ∼57, ∼76, ∼96, ∼130, ∼227, and ∼303 days are also detected in different energy bands. We discuss our results in light of previous observations and existing numerical models.

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Section: Introductionmentioning

confidence: 90%

Periodicities in the X-Ray Emission From the Solar Corona

Chowdhury

Jain

Awasthi

2013

ApJ

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“…Nevertheless, the solar wind is often more complex than was observed during the first Ulysses orbit (e.g., McComas et al, 2006). In this study, we use Wind (1995 -2004) and Advanced Composition Explorer (ACE) (1998 -2004), which are both close to the equatorial plane at about one AU.…”

Section: List Of Stream Interaction Regionsmentioning

confidence: 99%

Properties of Stream Interactions at One AU During 1995 – 2004

et al. 2006

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Abstract.A stream interaction region (SIR) forms when a fast solar stream overtakes a slow stream, leading to structure that evolves as an SIR moves away from the Sun. Based on Wind (1995 -2004) and ACE (1998ACE ( -2004 in situ observations, we have conducted a comprehensive survey of SIRs at one AU, including a separate assessment of the longer-lasting corotating interaction regions (CIRs) that recur on more than one solar rotation. In all there are 196 CIRs, accounting for about 54% of the 365 SIRs. The largest proportion of CIRs to SIRs (64%) appears in 1999, and the smallest proportion (49%) is in 2002. Over the ten years, the annual number of SIR events varies little, from 32 up to 45. On average, the occurrence rate of shocks at SIRs at one AU is about 24%. Seventy percent of the SIRs with shocks have only forward shocks, more than twice the percentage of SIRs with only reverse shocks. This preponderance of forward shocks is consistent with the deflections of forward and reverse shocks relative to the ecliptic plane. In order to help address the effect of SIRs and CIRs on geomagnetic activity, we determine the solar-cycle variation of the event duration, scale size, the change in velocity from slow stream to fast stream, and the solar-cycle variation of the maximum magnetic field, peak total perpendicular pressure, and other properties. These statistics also provide a baseline for future studies at other heliocentric distances and for validating heliospheric models.

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“…These expressions are for latitudes above ±40°and heliocentric distances within 4.5 AU. We chose a slightly higher latitude cut-off compared to the first orbit as the band of solar wind variability extends to higher latitudes for the current Ulysses orbit [McComas et al, 2006]. The reason for the 4.5 AU distance cut-off is that we have very little fast wind data past this distance.…”

Section: Smith and Baloghmentioning

confidence: 99%

Bulk properties of the slow and fast solar wind and interplanetary coronal mass ejections measured by Ulysses: Three polar orbits of observations

et al. 2009

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[1] We examined plasma and magnetic field observations from all three Ulysses polar orbits of the Sun to study the properties of the slow and fast solar wind and interplanetary coronal mass ejections (ICMEs). We derived equations to characterize the radial and latitudinal variations for these three types of heliospheric plasma and identify distinguishing features in their spatial variations. Most notably, the slow-wind proton temperature falls less rapidly with distance than does the fast wind, indicating a source of enhanced heating in the low-speed wind. After removing the radial variations from the measurements, only minor latitudinal gradients were identified. The fast wind has now been shown to be only weakly dependent on solar latitude for two successive solar minima. The spatial variations in the ICME properties do not differ significantly from the slow and fast solar wind, although the variability in their parameters is much larger. We also investigated solar cycle variations in the fast polar coronal hole (PCH) flows by comparing their properties measured over Ulysses' 1st and 3rd orbits. While the latitudinal gradients were similar, slight differences were observed in the radial dependence for the proton density and magnetic field strength. Also, a slight reduction in the proton speed at 1 AU, along with more significant decreases in the proton temperature, density, dynamic pressure, and magnetic field strength, was observed for the 3rd orbit relative to that for the 1st. These results are consistent with recent observations of weaker PCH flows for the current solar minimum.

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Ulysses observations of very different heliospheric structure during the declining phase of solar activity cycle 23

Cited by 49 publications

References 14 publications

Periodicities in the X-Ray Emission From the Solar Corona

Periodicities in the X-Ray Emission From the Solar Corona

Properties of Stream Interactions at One AU During 1995 – 2004

Bulk properties of the slow and fast solar wind and interplanetary coronal mass ejections measured by Ulysses: Three polar orbits of observations

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