Timo Pitkänen

Magnetic forces associated with bursty bulk flows in Earth's magnetotail

We present the first direct measurements of magnetic forces acting on bursty bulk flow plasma in the magnetotail. The magnetic forces are determined using Cluster multispacecraft measurements. We a ...

IMF dependence of the azimuthal direction of earthward magnetotail fast flows

Cluster magnetotail data together with ACE solar wind data from 2001 to 2009 are used to investigate the dependence of the azimuthal flow direction of earthward magnetotail fast flows on the interp ...

Evidence for the braking of flow bursts as they propagate toward the Earth

In this article we use energy conversion arguments to investigate the possible braking of flow bursts as they propagate toward the Earth. By using EJ data (E and J are the electric field and the cu ...

Response of magnetotail twisting to variations in IMF B-y : a THEMIS case study 1-2 January 2009

Theoretical considerations, observations, and simulations have shown that the B-y component of the interplanetary magnetic field (IMF) may cause twisting of the magnetotail. However, the fundamenta ...

Propagation of small size magnetic holes in the magnetospheric plasma sheet

Magnetic holes (MHs), characteristic structures where the magnetic field magnitude decreases significantly, have been frequently observed in space plasmas. Particularly, small size magnetic holes (SSMHs) which the scale is less than or close to the proton gyroradius are recently detected in the magnetospheric plasma sheet. In this study of Cluster observations, by the timing method, the minimum directional difference (MDD) method, and the spatiotemporal difference (STD) method, we obtain the propagation velocity of SSMHs in the plasma flow frame. Furthermore, based on electron magnetohydrodynamics (EMHD) theory we calculate the velocity, width, and depth of the electron solitary wave and compare it to SSMH observations. The result shows a good accord between the theory and the observation.

Detection of meteoroid hypervelocity impacts on the Cluster spacecraft: First results

We present the first study of dust impact events on one of the Earth-orbiting Cluster satellites. The events were identified in the measurements of the wide band data (WBD) instrument on board the ...

Oxygen ion response to proton bursty bulk flows

We have used Cluster spacecraft data from the years 2001 to 2005 to study how oxygen ions respond to bursty bulk flows (BBFs) as identified from proton data. We here define bursty bulk flows as per ...We have used Cluster spacecraft data from the years 2001 to 2005 to study how oxygen ions respond to bursty bulk flows (BBF:s) as identified from proton data. We here define bursty bulk flows as periods of proton perpendicular velocities more than 100 km/s and a peak perpendicular velocity in the structure of more than 200 km/s, observed in a region with plasma beta above 1 in the near-earth central tail region. We find that during proton BBF:s only a minor increase in the O+ velocity is seen. The different behavior of the two ion species is further shown by statistics of H+ and O+ flow also outside BBF:s: For perpendicular earthward velocities of H+ above about 100 km/s, the O+ perpendicular velocity is consistently lower, most commonly being a few 10 km/s earthward. In summary, O+ ions in the plasma sheet experience less acceleration than H+ ions and are not fully frozen in to the magnetic field. Therefore H+ and O+ motion is decoupled, and O+ ions have a slower earthward motion. This is particularly clear during BBF:s. This may add further to the increased relative abundance of O+ ions in the plasma sheet during magnetic storms. The data indicate that O+ is typically less accelerated in association with plasma sheet X-lines as compared to H+.

Potential of Earth Orbiting Spacecraft Influenced by Meteoroid Hypervelocity Impacts

Detection of hypervelocity impacts on a spacecraft body using electric field instruments has been established as a new method for monitoring of dust grains in our solar system. Voyager, WIND, Cassini, and STEREO spacecraft have shown that this technique can be a complementary method to conventional dust detectors. This approach uses fast short time changes in the spacecraft potential generated by hypervelocity dust impacts, which can be detected by monopole electric field instruments as a pulse in the measured electric field. The shape and the duration of the pulse strongly depend on parameters of the ambient plasma environment. This fact is very important for Earth orbiting spacecraft crossing various regions of the Earth’s magnetosphere where the concentration and the temperature of plasma particles change significantly. We present the numerical simulations of spacecraft charging focused on changes in the spacecraft potential generated by dust impacts in various locations of the Earth’s magnetosphere. We show that identical dust impacts generate significantly larger pulses in regions with lower electron density. We discuss the influence of the photoelectron distribution for dust impact detections showing that a small amount of energetic photoelectrons significantly increases the potential of the spacecraft body and the pulse duration. We also show that the active spacecraft potential control (ASPOC) instrument onboard the cluster spacecraft strongly reduces the amplitude and the duration of the pulse resulting in difficulties of dust detection when ASPOC is ON. Simulation of dust impacts is compared with pulses detected by the Earth orbiting cluster spacecraft in the last part of Section III.

The Delayed Time Response of Geomagnetic Activity to the Solar Wind

We investigate the lagged correlation between a selection of geomagnetic indices and solar wind parameters for a complete solar cycle, from 2000 to 2011. We first discuss the mathematical assumptio ...

Low frequency oscillatory flow signatures and high-speed flows in the Earth's magnetotail†

Using plasma sheet data from Cluster 1 spacecraft from 2001 till 2011, we statistically investigate oscillatory signatures in the plasma bulk flow. These periodic oscillations are compared to high- ...